Hazardous substance assessment – Styrene

This hazardous substance assessment was conducted according to both the former and amended Hazardous Products Regulations (HPR). Learn more about the HPR amendments and transition period.

Identification

Chemical name: 

Styrene

CAS #:

100-42-5

Chemical composition:

C₆H₅CHCH₂

Synonyms:

• Benzene, ethenyl-
• Cinnamene
• Phenylethene
• Styrolene

UN #:

2055

Pictogram(s):

Figure 1 – Text description

The symbol within the pictogram is an exclamation mark. This symbol indicates that hazardous products with this pictogram can cause certain health effects for example:

• skin irritation
• eye irritation
• skin sensitization

Figure 2 – Text description

The symbol within the pictogram is a black silhouette of a person's head and chest with a white star shape spreading out from the center of the chest. This symbol indicates that hazardous products with this pictogram can cause certain health effects for example:

• carcinogenicity
• specific target organ effects following single or repeated exposure
• reproductive toxicity

Figure 3 – Text description

The symbol within the pictogram is a flame with a line underneath it. This symbol indicates that hazardous products with this pictogram can ignite easily and burn rapidly if they are not stored and handled properly.

WHMIS classification

Health hazards:

• Acute Toxicity (Inhalation) – Category 4
• Carcinogenicity – Category 1B
• Aspiration Hazard – Category 1

Physical hazards:

• Flammable Liquids – Category 3

Health hazards

Acute Toxicity (Oral):

Does not meet criteria

Oral median lethal dose (LD₅₀): 2 650 milligrams per kilogram of body weight (mg/kg-bw) (rat, male)^{Reference 1}.

In an acute oral toxicity study, Wistar rats (5 per sex per dose) were fed orally with a single administration of styrene at 6 increasing dose levels (doses not specified). Animals were fasted for 15 hours prior to the treatment. Animals were observed for mortality and clinical signs for 14 days. Necropsies were performed on dead animals immediately after death or on surviving animals at the end of the study. LD₅₀ values of 2 650 mg/kg-bw and 3 050 mg/kg-bw were reported for male and female rats, respectively. Clinical signs included hypoactivity and staggering gait^{Reference 1}.

Another study reported an LD₅₀ of 5 000 mg/kg-bw in rats^{Reference 2}.

The available data do not meet the classification criteria for a category of Acute Toxicity (Oral).

Acute Toxicity (Dermal):

Does not meet criteria

Dermal LD₅₀: >2 000 mg/kg-bw (rat) ^{Reference 3}.

In a limit test in rats (5 per sex per dose) exposed to a dose of 2 000 mg/kg-bw of styrene for 24 hours under semi-occlusive conditions, no deaths or clinical signs were observed^{Reference 3}.

A non-guideline study was conducted in rabbits (1 to 2 per dose) exposed to a dose of 1, 3.3, 10 or 20 grams of styrene per kilogram of body weight (g/kg-bw) for 24 hours. No mortality was observed and the LD₅₀ was greater than 20 g/kg-bw. At the highest dose, 1 of the animals had temporary paralysis and at the 2 highest doses narcosis was reported^{Reference 4}.

The available data do not meet the classification criteria for a category of Acute Toxicity (Dermal).

Acute Toxicity (Inhalation – Gases):

Not applicable

Styrene is not a gas. The classification criteria for Acute Toxicity (Inhalation – Gases) do not apply to this substance.

Acute Toxicity (Inhalation – Vapours):

Category 4

Inhalation median lethal concentration (LC₅₀): 11.8 milligrams per litre (mg/L) (rat)^{Reference 5}.

In non-guideline acute toxicity studies in mice and rats exposed for 4 hours (rats) or 2 hours (mice), the LC₅₀ for rats was 11.8 mg/L and was 21.0 mg/L for mice. No other study details were available^{Reference 5}.

A non-guideline study in male mice (10 per dose) exposed to a concentration of 0, 0.17, or 0.68 mg/L of styrene vapour for 2 to 6 hours reported no mortality or adverse clinical signs. The LC₅₀ was greater than 0.68 mg/L^{Reference 3}.

Another non-guideline study in male mice (n=50) exposed to a concentration of 2.13 mg/L of styrene vapour for 6 hours reported an LC₅₀ greater than 2.13 mg/L and clear signs of liver damage, specifically a 26% increase in liver weights, centrilobular hepatocellular necrosis, and increased hepatocyte cell proliferation^{Reference 3}. A similar study also reported liver effects described as centrilobular coagulative necrosis^{Reference 3}.

The available data meet the classification criteria for Acute Toxicity (Inhalation) – Category 4 [HPR 8.1.1(1)].

Acute Toxicity (Inhalation – Dusts and Mists):

No data available

No data are available to determine whether styrene meets the classification criteria for a category of Acute Toxicity (Inhalation – Dusts and Mists).

Skin Corrosion / Irritation:

Does not meet criteria

In a study involving 9 human volunteers, no skin irritation was reported from dipping 1 hand fully into liquid styrene for 10 to 30 minutes^{Reference 6}.

Several animal studies reported in the literature disclose that styrene is a skin irritant^{Reference 2Reference 3Reference 7}; however, these studies provide no scores or details about study methods.

The available data do not meet the classification criteria for a category or subcategory of Skin Corrosion / Irritation.

Serious Eye Damage / Eye Irritation:

Does not meet criteria

In a study conducted similarly to the Organisation for Economic Co-operation and Development (OECD) Test Guideline 405, 0.1 millilitre (mL) of undiluted styrene was applied to the conjunctival sac of 4 rabbits. Eyes were scored for irritation at 4 hours and 1, 2, and 3 days post-treatment. Mean scores for each animal calculated following gradings at 24, 48 and 72 hours after instillation of the substance were: 0/4 for 3 animals and 0.33/4 for 1 animal for corneal opacity; 0/2 for all 4 animals for iritis; 0.67/3 for 1 animal, 0.33/3 for 1 animal and 1/3 for 2 animals for conjunctival redness; and 0.33/4 for 3 animals and 1/4 for 1 animal for chemosis. The mean scores do not meet classification criterion (c)(i) or (ii) of the Table to subsection 8.3.2(1) of the HPR nor of those of the Table to subsection 8.3.2(3) of the HPR. For 2 of the animals, scores were also taken at subsequent time points, both demonstrating reversal of effects (scores of 0 for all parameters) at 7 or 10 days^{Reference 8}.

The available data do not meet the classification criteria for a category or subcategory of Serious Eye Damage / Eye Irritation.

Respiratory Sensitization:

Does not meet criteria

Despite widespread industrial exposure to styrene, there are only a few published cases of asthma in which it may be implicated, and in each case the evidence for a role for styrene exposure in inducing the asthma is not convincing^{Reference 9}.

The available data do not meet the classification criteria for a category or subcategory of Respiratory Sensitization.

Skin Sensitization:

Does not meet criteria

In humans, a skin sensitization study in 303 individuals exposed to a concentration of 5% of styrene in petrolatum identified no signs of skin sensitization^{Reference 10}.

The skin sensitization potential of styrene was evaluated in a guinea pig maximization test (n=15 animals). Induction was performed via intradermal or topical application of a 10% (weight per volume, w/v) or a 20% (w/v) styrene solution, respectively, followed by challenge with 2% (w/v) styrene in acetone. Skin sensitization was not observed in any of the animals tested^{Reference 11}.

In another guinea pig maximization test, 10 guinea pigs were induced with a concentration of 5% styrene (epicutaneously). Challenges were then performed with a concentration of 1% styrene. Only 1 out of 10 animals had a positive reaction at 24 hours post-challenge^{Reference 3}.

The available data do not meet the classification criteria for a category or subcategory of Skin Sensitization.

Germ Cell Mutagenicity:

Does not meet criteria

In vivo:

A micronucleus assay, conducted similarly to OECD Test Guideline 474, was carried out in mice exposed via whole-body inhalation to a concentration of 0.75 or 1.5 mg/L of styrene vapour for 6 hours per day for 1, 3, 7, 14 or 21 consecutive days. There was no evidence of clastogenicity at any concentration or exposure interval tested ^{Reference 12}.

A sister chromatid exchange assay was conducted in mice exposed via inhalation to a concentration of 0, 125, 250 or 500 parts per million (ppm) (0, 0.53, 1.06, or 2.125 mg/L, respectively) of styrene vapour for 6 hours per day for 14 days. A small concentration-related increase in sister chromatid exchange frequency was found in lymphocytes from the spleen, peripheral blood and the lungs; however, no significant concentration-related increases were found in the frequency of chromosomal aberrations in the cultured splenocytes or lung cells and no significant increase was observed in micronucleus frequencies in splenocytes or erythrocytes in peripheral blood smears^{Reference 3Reference 13}.

An unscheduled deoxyribonucleic acid (DNA) synthesis test similar in design to OECD Test Guideline 486 was conducted in female mice (5 per dose) exposed via inhalation to a single dose of 125 or 250 ppm of styrene for 6 hours. No genotoxic effects were reported ^{Reference 14}.

A bone marrow chromosomal aberration test similar in design to OECD Test Guideline 475 was conducted in male mice orally administered a dose of 500 milligrams of styrene per kilogram of body weight per day (mg/kg-bw/day) for 4 days or a dose of 200 mg/kg-bw/day for 70 days. No mutagenic effects were observed^{Reference 15}.

A study similar in design to OECD Test Guideline 475 (Mammalian Bone Marrow Chromosomal Aberration Test) conducted in mice gavaged with a single dose of 0, 500 or 1 000 mg/kg-bw of styrene proved negative for increased chromosomal aberrations^{Reference 16}.

Several additional in vivo studies are available in rats, mice and hamsters with mixed negative and positive results across various types of assays, mostly via inhalation and oral exposures^{Reference 3}.

In vitro:

Ames assays in Salmonella typhimurium TA 98, TA 100, TA 1535, TA 1537 and TA 1538 strains provided mostly negative results with and without metabolic activation [17-20]. Positive results in the strain TA 100 were observed in 1 study after some types of metabolic activation^{Reference 19}.

Several additional in vitro studies are available from various types of assays with mixed negative and positive results^{Reference 3}.

Exposure to styrene also induced a dose-dependent increase in sister chromatid exchanges in human whole-blood lymphocyte cultures without exogenous metabolic activation systems^{Reference 21}.

The available data do not meet the classification criteria for a category or subcategory of Germ Cell Mutagenicity.

Carcinogenicity:

Category 1B

Styrene has been reviewed by the International Agency for Research on Cancer and has been classified as probably carcinogenic to humans (Group 2A) [22-24] . It has also been reviewed by the National Toxicology Program and has been determined to be reasonably anticipated to be a human carcinogen, based on limited evidence of carcinogenicity from studies in humans, sufficient evidence of carcinogenicity from studies in experimental animals, and supporting data on mechanisms of carcinogenesis [25] . The American Conference of Governmental Industrial Hygienists has classified styrene as A4, not classifiable as a human carcinogen^{Reference 26}.

Workers exposed to styrene showed increased mortality and incidence of cancer of the lymphohematopoietic system, increased levels of DNA adducts and genetic damage in lymphocytes^{Reference 25}.

A study equivalent in design to OECD Test Guideline 453 (Combined Chronic Toxicity/Carcinogenicity Studies) was conducted in CD-1 mice (70 per sex per dose) exposed via whole-body inhalation to styrene vapours at a concentration of 0.09, 0.18, 0.35 or 0.69 mg/L (20, 40, 80 and 160 ppm, respectively) for 6 hours per day, 5 days per week for 98 weeks in females and 104 weeks in males. The incidence of bronchioalveolar adenomas was in males exposed to a concentration of ppm (35 of 50 animals), 80 ppm (30 of 50 animals) or 160 ppm (33 of 50 animals) of styrene vapours compared to unexposed concurrent controls. In females, the incidence of bronchioloalveolar adenomas in the groups exposed to a concentration of 20 ppm (16 of 50 animals), 40 ppm (16 of 50 animals) or 160 ppm (24 of 50 animals) of styrene vapours, but not to a concentration of 80 ppm (11 of 50 animals), for 98 weeks was significantly increased as well as the incidence of bronchioloalveolar carcinoma in the highest concentration group (7 of 50 animals) compared to unexposed concurrent controls^{Reference 27}.

A non-guideline study was conducted in B6C3F1 mice (50 per sex per dose) gavaged with styrene at a dose of 150 or 300 mg/kg-bw/day for 13 weeks. Increased incidences of alveolar and bronchiolar neoplasms were observed in male mice compared to controls in both dose groups. In female mice, a significant positive association between dose and the incidence of hepatocellular adenomas was also reported ^{Reference 28}.

The available data meet the classification criteria for Carcinogenicity – Category 1B [HPR 8.6.1].

Reproductive Toxicity:

Does not meet criteria        

An OECD Test Guideline 416-compliant (2-Generation Reproduction Toxicity) study was conducted in rats (25 per sex per dose) exposed to styrene via whole-body inhalation at a concentration of 0, 0.21, 0.64 or 2.13 mg/L (F₀ and F₁ animals) for 6 hours per day. F₀ and F₁ animals were treated for 70 days, with treatment continued for F₀ and F₁ females throughout mating and gestation through to gestation day 20. On lactation days 1 through 4, the F₀ and F₁ females received styrene via oral gavage at a dose of 66, 117 or 300 mg/kg-bw/day. These doses were calculated to provide the same maternal blood peak concentration as provided by the inhalation exposure. There were no indications of adverse effects on reproductive performance in either the F₀ or F₁ generation. Male and female mating and fertility indices, pre-coital intervals, spermatogenic endpoints, reproductive organ weights, lengths of estrous cycle and gestation, live litter size and postnatal survival were similar in all exposure groups. There was an exposure-related reduction in mean body weight gain of the F₂ offspring from the mid- and high-exposure groups and an overall pattern of slightly delayed development evident in the F₂ offspring only from the high-exposure group. This developmental delay included reduced body weight (which continued through to day 70) and slightly delayed acquisition of some physical landmarks of development. In the F₀ and F₁ generations, degenerative effects in the olfactory epithelium for the high-exposure group were reported^{Reference 29}.

A non-guideline study was conducted in rats (10 males and 20 females per dose) administered a concentration of 125 or 250 ppm of styrene continuously in drinking water (the 250-ppm exposure being equivalent to a dose of 18 mg/kg-bw/day for males or of 23 mg/kg-bw/day for females) over a 2-year period. Reduced pup survival from the high-dose group of F₀ parents 21 days after birth and from F₁ parents from day 1 to 14 after birth was observed, with only slight reductions in maternal body weight gain. No other significant clinical signs nor systemic or developmental effects were reported^{Reference 3Reference 10}.

A study similar in design to OECD Test Guideline 422 (Combined Repeated Dose Toxicity Study with the Reproduction/Developmental Toxicity Screening Test) was conducted in male rats (n=21) gavaged with a dose of 200 or 400 mg/kg-bw/day of styrene for 6 days per week for 60 days. At a dose of 400 mg/kg-bw/day, a significant decrease in epididymal sperm count and marked histopathological changes in the testes (shrunken seminiferous tubules with damaged nuclei and an absence of sperm in the lumen of some tubules) were reported^{Reference 3}.

Another study similar in design to OECD Test Guideline 422 was conducted in mice administered a dose of 1.2 or 12 mg/kg-bw/day of styrene in drinking water for 4 weeks. No adverse signs of toxicity were observed ^{Reference 3}.

A secondary source describes several reproductive toxicity studies on styrene. A well-conducted 90-day repeated exposure study in rats revealed no evidence of testicular effects at airborne concentrations of styrene of up to 1 500 ppm. In a well-conducted developmental toxicity study, inhalation exposure to styrene at concentrations up to 600 ppm in rats and rabbits was not associated with significant effects on the fetus. In other studies, delayed fetal development or increases in minor developmental anomalies in rat and mouse were only observed at doses causing maternal toxicity. A range of epidemiological studies focusing on developmental effects have been conducted but most of these investigations have been too small to be conclusive. Overall, the authors of this secondary source concluded that there is no evidence of an effect of styrene on human reproduction^{Reference 7}.

A non-guideline study in rats (n=29 to 30) gavaged with a dose of 180 or 300 mg/kg-bw/day of styrene during gestational days 6 to 15 reported maternal toxicity as indicated by decreased body weight gain and reduced food consumption at both doses. No teratogenic effects were observed^{Reference 3}.

In an OECD Test Guideline 414-compliant study, 30 rats per dose were exposed via inhalation to styrene vapour at a concentration of 1.278 or 2.556 mg/L for 7 hours per day on gestation days 6 to 15. No significant differences in litter sizes were observed nor increases in skeletal variations. In addition, 20 rabbits per dose were exposed at the same concentrations of styrene vapour for 7 hours per day from gestation day 6 to 18. There were also no statistically significant developmental effects in the treated groups compared to controls. Skeletal variations were also within the range of historical control groups^{Reference 3}.

Additional reproductive and developmental studies in rodents are available with mixed negative and positive results reported^{Reference 3Reference 7Reference 30}. Many of these studies are of poor quality and are insufficient for use in classification evaluation. Several studies have reported reproductive or developmental effects in rodents at maternally toxic doses. An expert panel convened by the Center for the Evaluation of Risks to Human Reproduction could not determine if developmental effects in animal studies were a direct effect of exposure to styrene or were secondary to maternal toxicity; this panel expressed negligible concern for reproductive and developmental toxicity of styrene in humans ^{Reference 30}.

The available data do not meet the classification criteria for a category or subcategory of Reproductive Toxicity.

Specific Target Organ Toxicity – Single Exposure:

Does not meet criteria

Oral Route of Exposure: In an acute oral toxicity study, Wistar rats (5 per sex per dose) were fed orally with a single administration of styrene at 6 increasing dose levels (doses not specified). LD₅₀ values of 2 650 mg/kg-bw and 3 050 mg/kg-bw were reported for male and female rats, respectively, with clinical signs of hypoactivity and staggering gait ^{Reference 1}.

Dermal Route of Exposure: In a limit test in rats (5 per sex per dose) exposed to a dose of 2 000 mg/kg-bw of styrene for 24 hours under semi-occlusive conditions, no deaths or clinical signs were observed ^{Reference 3}.

A non-guideline study was conducted in rabbits (1 to 2 per dose) exposed to a dose of 1, 3.3, 10 or 20 g/kg-bw of styrene for 24 hours. No mortality was observed and the LD₅₀ was greater than 20 g/kg-bw. At the highest dose, 1 of the animals had temporary paralysis and at the 2 highest doses narcosis was reported ^{Reference 4}. Narcotic effects in animals alone do not normally warrant classification.

Inhalation Route of Exposure: In a human study, exposure to styrene vapour at a concentration of 100 ppm generated a strong odour but was well-tolerated; at styrene concentrations of 200 to 400 ppm, the odour was objectionable; and at a styrene concentration of 600 ppm or more, a very strong odour with strong eye and nasal irritation resulted^{Reference 2}. As the subjective observations were not accompanied by objective measures indicating local cytotoxicity, the irritant effects do not result in classification.

A non-guideline study in male mice (10 per dose) exposed to a concentration of 0, 0.17 or 0.68 mg/L of styrene vapour for 2 to 6 hours reported no mortality or adverse clinical signs. The LC₅₀ was greater than 0.68 mg/L ^{Reference 3}.

Another non-guideline study in male mice (n=50) exposed to a concentration of 2.13 mg/L of styrene vapour for 6 hours reported an LC₅₀ greater than 2.13 mg/L and clear signs of liver damage, specifically a 26% increase in liver weights, centrilobular hepatocellular necrosis, and increased hepatocyte cell proliferation. Deaths associated with the liver damage were observed at day 5 post-exposure ^{Reference 3}. The liver damage does not result in classification given this association with mortality. A similar study also reported liver effects described as centrilobular coagulative necrosis^{Reference 3}.

The available data do not meet the classification criteria for a category of Specific Target Organ Toxicity – Single Exposure.

Specific Target Organ Toxicity – Repeated Exposure:

Does not meet criteria

Oral Route of Exposure: In a repeated-dose toxicity study, female rats (10 per dose) were fed a dose of 66.7, 133, 400 or 667 mg/kg-bw/day of styrene for 5 days per week for 6 months. At doses up to 133 mg/kg-bw/day, no effects were observed. At higher doses, depressed growth and increased liver and kidney weights were noted^{Reference 2}. The results of this study do not meet the classification criteria of a category of this hazard class.

Another study, that was non-guideline, was conducted in rats (50 per sex per dose) administered styrene at a dose of 500 mg/kg-bw/day for 5 days per week for 103 weeks, or at a dose of 1 000 or 2 000 mg/kg-bw/day for 78 weeks. Male rats had a lower mean body weight compared to controls while the body weights of females were less affected by exposure. Mortality of both sexes was significantly increased in the 2 000 mg/kg-bw/day-dose group compared to controls^{Reference 3}. The dose at which mortality occurred is outside the guidance values for classification in a category of this hazard class. 

In another chronic study, styrene was orally administered to 50 mice per sex and per dose at a dose of 150 or 300 mg/kg-bw/day, for 5 days per week over 78 weeks, followed by a non-exposure period of 13 weeks. At a dose of 300 mg/kg-bw/day, significant mortality was seen in male, but not in female, mice. No other significant adverse signs of toxicity were reported that would warrant classification ^{Reference 3}. The dose at which mortality occurred is outside the guidance values for classification in a category of this hazard class.

In a short-term study, 10 mice per sex and per dose were orally administered styrene at a dose of 10, 100, or 200 mg/kg-bw/day for 5 days. No significant adverse signs of toxicity were observed that would warrant classification. Slight focal crowding of Clara cells in the epithelium of the terminal bronchiole in the lungs was noted (dose level unspecified) ^{Reference 3}. The results of this study do not meet the classification criteria of a category of this hazard class.

A non-guideline study was conducted in male Long-Evans rats gavaged with a dose of 200 mg/kg-bw/day of styrene for 12 weeks (n=6), of 300 mg/kg-bw/day for 3 weeks (n=6), of 400 mg/kg-bw/day for 3 (n=6) or 6 weeks (n=6) or of 800 mg/kg-bw/day for 3 weeks (n=11). At all tested doses, loss of outer hair cells (OHC) and inner hair cells (IHC) occurred and cochlear sensitivity was affected^{Reference 31}. The effect of these results on actual hearing loss was not clear and, except at doses of 300 and 400 mg/kg-bw/day over 3 weeks, the doses at which effects occurred are outside the guidance values for classification in a category of this hazard class.

Dermal Route of Exposure: No data available

Inhalation Route of Exposure:

In humans: Styrene has been investigated in several human studies evaluating the relationship between occupational exposure and hearing loss, with some of these studies suggesting an association^{Reference 32Reference 33Reference 34Reference 35Reference 36Reference 37Reference 38} while others do not^{Reference 39Reference 40Reference 41Reference 42}. It is unclear whether styrene exposure alone causes hearing loss in humans.

In animals: In 1 study, exposure to a concentration of 800 ppm of styrene vapour (3.4 mg/L) for 3 weeks caused marked hearing loss in rats as assessed by behavioral (conditioned avoidance) and electrophysiologic (brainstem auditory-evoked response) methods. In that study, rats (12 per concentration) were exposed to a concentration of 0, 800, 1 000 or 1 200 ppm of styrene vapour via whole-body inhalation for 14 hours per day for 3 weeks ^{Reference 43}. Since the exposure time was 14 hours per day as opposed to the 6 hours per day of the guidance values, the study cannot be used for classification.

Other studies have also reported auditory system effects due to styrene vapour exposure. In one of these studies, groups of 14 rats per dose were exposed to a concentration of 0, 50, 200 or 800 ppm (0, 0.21, 0.85 or 3.41 mg/L, respectively) of styrene vapour for 6 hours per day, 5 days per week for 13 weeks. Histopathological investigations revealed lesions in the organ of Corti of rats in the 800-ppm exposure group. Hearing thresholds, as measured by auditory nerve response, were also elevated by 40 decibels (dB) at frequencies of 16, 25 and 30 kHz at the 800-ppm exposure concentration, indicating ototoxic effects. The no-observed-adverse-effect-concentration (NOAEC) for ototoxicity was 200 ppm, with evidence of auditory system damage and impairment at a concentration of 800 ppm (3.41 mg/L)^{Reference 3}. The effects occurred at a concentration outside the guidance values for classification in a category of this hazard class.

In another inhalation study (whole-body), adult male rats were exposed to a concentration of 1 600 ppm of styrene vapour (mg/L) for 6 hours per day for 5 weeks in flow-through chambers. Inhalation exposure resulted in hearing dysfunction only in the mid-frequency range and spared function at lower and higher frequencies^{Reference 44}. The concentration at which hearing effects were observed is above the guidance values for classification in a category of this hazard class.

In another inhalation study (whole-body), groups of 8 male rats were exposed to a concentration of 0, 500, 650, 850, 1000 or 1500 ppm of styrene vapour for 6 hours per day, 5 days per week for 4 weeks ^{Reference 45}. The permanent threshold shifts (PTS) from audiometric testing in this study were significantly elevated at exposure concentrations of 850 ppm and above. At a concentration of 850 ppm, there was a large PTS over the frequency range of 16 to 20 kilohertz (kHz), but no significant hearing loss noted at lower exposure concentrations. There were no effects on inner hair cells (IHC) in treated or control animals. Animals treated with a concentration of 650 ppm of styrene vapour or above showed loss of outer hair cells (OHC) in all three rows of cells examined, particularly those of the third row.

Overall, this study demonstrates that styrene is ototoxic in rats following inhalation exposure at vapour concentrations of 650 ppm and above, based on loss of OHC, with a clear NOAEC being identified at 500 ppm. However significant hearing loss was evident only at 850 ppm and above. Given the lack of significant hearing loss at the highest concentration tested that is within guidance values for this hazard class, namely 650 ppm, the adversity of the loss of OHC at that concentration on rat inner ear function is unclear.

Similar results were found in another inhalation study (whole-body), with a focus on activity and noise level ^{Reference 46}. Groups of 4 male rats were exposed under active conditions to a concentration of 0, 300, 400, 500 or 600 ppm of styrene vapour, without application of noise, for 6 hours per day, 5 days per week for 4 weeks. In addition, groups of 6 rats were exposed under active conditions to noise only (sound pressure level of 86.2 dB; octave band centered at 8 kHz), to styrene vapour only (at 400 ppm), to noise and styrene vapour, or to neither. The rats ran 2 minutes of every 3 minutes in a running wheel to maintain active conditions. Audiometric testing was performed prior to styrene exposure and at 6 weeks after the exposure period. Following audiometric testing, the rats were assessed for damage to the IHC and OHC of the organ of Corti.

Significant hearing loss, as determined by PTS, was measured at exposure concentrations of 500 ppm of styrene vapour and above. The activity of the rats was hypothesized to increase uptake of styrene through an increase in pulmonary ventilation and in cardiac output. Furthermore, combined exposure to noise and styrene was identified to increase the loss of OHC relative to exposure to styrene alone, whereas the combined exposure did not increase hearing loss relative to exposure to noise alone. As the inhalation guidance values for this hazard class are concentration-based, the effect levels in this study, which reflect increased uptake, cannot be used for classification.

In another repeated-dose toxicity study, 10 rats per dose were exposed to a concentration of 0 or 1.299 mg/L of styrene vapour via whole-body inhalation for 6 hours per day, 5 days per week for 12 weeks. A 30% reduction of large amacrine cells, which are believed to play a role in colour vision, was reported upon post-mortem examination of treated rats relative to controls. Levels of glutathione, an antioxidant that protects the eyes from chemical and oxidative stress, were also found to be 28% lower in the retinas of exposed animals compared to controls^{Reference 31}. The study suggests styrene may cause ocular toxicity via inhalation; however, the severity of the ocular effects is unclear as is their toxicological relevance.

A non-guideline study in female CD-1 mice exposed to styrene at a vapour concentration of 0, 172 or 688 milligrams per cubic metre (mg/m³) (0, 40 or 160 ppm, respectively) for 6 hours per day, 5 days per week for 20 exposure days, over a 4-week period, reported degenerative lesions in mouse lung Clara cells, as indicated by vacuolar cell degeneration and necrosis at the highest concentration tested. Similar effects were not seen in rats in the same study ^{Reference 47}. This study does not meet classification criteria for a category of this hazard class. The mechanism for the lung response in mice is largely inoperable in humans ^{Reference 28}. It has been proposed as part of a non-genotoxic mode of action for mouse lung tumors ^{Reference 47}, and thus, to the extent that it is operable in exposed workers, is considered classified for under the Carcinogenicity – Category 1B classification.

In another repeated-dose inhalation study, mice were exposed to styrene vapour at a concentration of 50, 100, 150 or 200 ppm (0.21, 0.42, 0.63 or 0.85 mg/L, respectively) for 6 hours per day, 5 days per week for 13 weeks and rats were exposed to a concentration of 50, 200, 500 or 1 000 ppm of styrene vapour (0.21, 0.85, 2.1 or 4.3 mg/L, respectively) for 6 hours per day, 5 days per week for 1 year. In mice exposed to a concentration of 100, 150 or 200 ppm of styrene, moderate to severe atrophy of the olfactory epithelium, dilatation and hypertrophy of Bowman's glands, and eosinophilic inclusions in respiratory and transitional epithelium were reported in nearly every animal examined. When these changes were marked in mice, viable olfactory epithelium remained in the nasal mucosa. A few mice exposed to a styrene concentration of 50 ppm exhibited similar effects, but to a lesser degree. For the 1-year rat exposure, similar findings were reported; however, the effects appeared to be less severe and damage to the olfactory epithelium was milder^{Reference 48}. Study details are insufficient for classification evaluation. Other studies have also reported effects on the olfactory epithelium, hypertrophy of thyroids, or decreased organ weights and adipose tissue due to chronic styrene vapour exposure ^{Reference 31}. However, the severity of effects is unclear or the concentrations at which these effects occurred were either unspecified or too high to meet classification criteria for a category of this hazard class

A 2-week study was conducted in mice of different strains (20 per sex per dose) exposed to styrene vapour at a concentration of 0.060, 0.248, 1.064 or 2.21 mg/L (measured) for 6 hours per day, 5 days per week. Mortality in CD1 mice was seen at a concentration of 1.064 mg/L (1 male, 7 females) and at 2.21 mg/L (7 males, 2 females). For B63F1 mice, mortality was seen at a concentration of 1.064 mg/L (1 male, 10 females) and at 2.21 mg/L (8 males, 0 females). At the 2 highest concentrations, clinical signs included lethargy, shallow breathing and unsteady gait. Changes in the lungs were apparent and included leukocytosis and single cell necrosis^{Reference 49}. Despite the elevated mortality observed in some of the mouse strains tested, the duration of the study is relatively short for a repeated-dose study and there is insufficient information for use in a classification evaluation.

A non-guideline study was conducted in 4 different murine strains (B6C3F1, C57BL/6, Swiss and DBA/2) exposed to styrene vapour at a concentration of 0, 125, 250 or 500 ppm (0, 0.53, 1.1 or 2.1 mg/L, respectively) for 6 hours per day for 4 days. For each concentration tested, 20 animals per sex and strain were used. Mortality, increased liver weights and hepatocellular necrosis were observed in the 250- and 500-ppm exposure groups of all strains tested. In B6C3F1 male mice, 14 out of 20 animals died at a concentration of 250 ppm, and in Swiss male mice, 10 out of 20 animals died at a concentration of 500 ppm. In the other strains tested, mortality also occurred but in smaller numbers ^{Reference 50}. Despite the elevated mortality observed in the mouse strains tested, the duration of the study is relatively short for a repeated-dose study and there is insufficient information for use in a classification evaluation.

The available information regarding the ototoxicity of styrene vapour in humans is conflicting and based on poorly reported epidemiological studies with several inherent confounding factors; the information from these studies is insufficient to use for classification evaluation. Moreover, the available animal studies cannot be used for classification, provided effect levels at concentrations outside the guidance values, or present effects of unclear adversity, and thus their results do not meet classification criteria for a category of this hazard class.

The available data do not meet the classification criteria for a category of Specific Target Organ Toxicity – Repeated Exposure.

Aspiration Hazard:

Category 1

Styrene is a liquid hydrocarbon with a kinematic viscosity of 0.77 square millimetre per second (mm²/s) at 25°C^{Reference 3}. At a temperature of 40°C, the viscosity would be even lower, and thus criterion 1(b) of the Table to section 8.10.1 of the HPR is met.

The available data meet the classification criteria for Aspiration Hazard – Category 1 [HPR 8.10.1].

Biohazardous Infectious Materials:

Not applicable

Styrene is not a microorganism, protein, or nucleic acid.

Physical hazards   

Explosives:

Not evaluated
Explosives are excluded from the HPA and its regulations. Explosives are regulated under the Explosives Act. For more information, visit Natural Resources Canada.

Flammable Gases:

Not applicable

Styrene is not a gas. The classification criteria for Flammable Gases do not apply to this substance.

(Flammable) Aerosols:

Not evaluated

Classification of a hazardous product in the Flammable Aerosols or Aerosols hazard class is product dependent.

Oxidizing Gases:

Not applicable

Styrene is not a gas. The classification criteria for Oxidizing Gases do not apply to this substance.

Gases Under Pressure:

Not applicable

Styrene is not a gas. The classification criteria for Gases Under Pressure do not apply to this substance.

Flammable Liquids:

Category 3

The flashpoint of styrene is 31°C (closed cup)^{Reference 51}.

The available data meet the classification criteria for Flammable Liquids – Category 3 [HPR 7.6.1(2)].

Flammable Solids:

Not applicable

Styrene is not a solid. The classification criteria for Flammable Solids do not apply to this substance.

Self-reactive Substances and Mixtures:

Does not meet criteria

Styrene has a boiling point of 145.3°C ^{Reference 51}. Self-reactive substances and mixtures must have a self-accelerating decomposition temperature of ≤75°C to meet the minimum classification in a category of this hazard class [HPR 7.8.1(3)].

The available data do not meet the classification criteria for a category of Self-reactive Substances and Mixtures.

Pyrophoric Liquids:

Does not meet criteria

Styrene has an auto-ignition temperature of 490°C^{Reference 51}. Pyrophoric liquids react at room temperature.

The available data do not meet the classification criteria for a category of Pyrophoric Liquids.

Pyrophoric Solids:

Not applicable

Styrene is not a solid. The classification criteria for Pyrophoric Solids do not apply to this substance.

Self-heating Substances and Mixtures:

Does not meet criteria

Styrene has an auto-ignition temperature of 490°C^{Reference 51}, which is well above the maximum spontaneous ignition temperature of 50°C for classification [HPR 7.11.1(3)].

The available data do not meet the classification criteria for a category of Self-heating Substances and Mixtures.

Substances and Mixtures Which, in Contact with Water, Emit Flammable Gases:

Excluded from classification

Styrene has a chemical structure that does not contain metals or metalloids and is, therefore, excluded from classification [HPR 7.12.1(1)(a)].

Oxidizing Liquids:

Does not meet criteria

Styrene is incapable of reacting exothermically with combustible materials^{Reference 3}.

The available data do not meet the classification criteria for a category of Oxidizing Liquids.

Oxidizing Solids:

Not applicable

Styrene is not a solid. The classification criteria for Oxidizing Solids do not apply to this substance.

Organic Peroxides:

Not applicable

Styrene is not an organic peroxide. The classification criteria for Organic Peroxides do not apply to this substance.

Corrosive to Metals:

Does not meet criteria

Styrene is not corrosive to stainless steel (e.g., types 301, 304, 316 and 347, the 400 series and Carpenter 20Cb-3), aluminum (types 3003 and Cast B-356), carbon steel (e.g., types 1010, 1020, 1075 and 1095), ductile cast iron, zinc-plated steel or galvanized steel, nickel-base alloy, Hastelloy C-22, naval brass or naval bronze, tantalum or titanium^{Reference 52Reference 53}.

The available data do not meet the classification criteria for a category of Corrosive to Metals.

Combustible Dusts:

Not applicable.

Styrene is not a solid. The classification criteria for Combustible Dusts do not apply to this substance.

Simple Asphyxiants:

Not applicable

Styrene is not a gas. The classification criteria for Simple Asphyxiants do not apply to this substance.

Pyrophoric Gases:

Not applicable

Styrene is not a gas. The classification criteria for Pyrophoric Gases do not apply to this substance.

Chemicals Under Pressure:

Not evaluated

Classification of a hazardous product in the Chemicals Under Pressure hazard class is product dependent.

Regulatory and other information

Regulatory information:

Hazardous substance assessments are prepared by Health Canada as educational and information resources. Under the HPA, suppliers of hazardous products must, upon the sale or importation of a hazardous product, provide a safety data sheet and label that meet the requirements set out in the HPR.   

Other information:

The information and classifications contained in these hazardous substance assessments are based on publicly available sources, such as peer-reviewed literature or reports by international bodies. New information, including proprietary information, could have an impact on the classification of substances or hazardous products containing them. It is the responsibility of the supplier to ensure the accuracy, sufficiency and reliability of their hazardous product classifications.  

Last updated:

2024

Prepared by:

Workplace Hazardous Materials Bureau, Health Canada

References