Appendix 8

Development of the Chronic Screening Level for Carbonyl Sulfide
Although the health effects data for carbonyl sulfide (COS) are very limited, a series of studies 

(ref 1,2,3) conducted by the National Toxicology Program have shown that the major concern 

regarding exposure to COS is its potential for neurotoxicity. These studies have shown 

consistently and at the same range of COS concentrations that the brain is a target organ for COS 

toxicity. A summary of the effects and levels at which they are observed is presented below. 

Since health appropriate effects benchmarks have not been derived by our preferred sources of 

dose-response data including IRIS, ATSDR and Cal EPA, the EPA has used the data from the 

above mentioned studies to derive a chronic screening benchmark level for COS. The rationale 

for this screening analysis is presented below. 

A chronic screening level of 163 ug/m3was developed for carbonyl sulfide (COS) from a No 

Observed Adverse Effects Level (NOAEL) of 200 ppm based on brain lesions and 

neurophysiological alterations in rodents as cited in the references below. 

The NOAEL value of 200 ppm was converted to a value of 490,000 ug/m3as follows:

1 ppm of COS = 2.45 mg/m3(Molecular Weight = 60.08)

200 ppm x 2.45 mg/m3= 490 mg/m3= 490,000 ug/m3

A total uncertainty factor (UF) of 3,000 was applied to this effect level: 10x for extrapolation for 

interspecies differences, 10x for consideration of intraspecies variability, 10x for extrapolation 

from subchronic to chronic duration, and 3x for database insufficiencies. 

10 x 10 x 10 x 3 = 3,000

(490,000 ug/m3) / 3,000 = 163 ug/m3

Summary of Findings from NTP Studies on Carbonyl Sulfide (COS)

Experimental Design:

Several studies were conducted using F344 rats exposed to carbonyl sulfide (COS) by inhalation 

at various exposure concentrations and durations. Details are available in the references cited 

below. 

Results Synopsis:

Pathology:

Brain lesions at 400 and 500 ppm after exposure for 2 weeks. NOAEL = 300 ppm 

[Reference 1]

Brain lesions at 400 ppm after exposure for 12 weeks. NOAEL = 300 ppm [Reference 1, 

Reference 3]

Biochemistry [Reference 1]:
Inhibited cytochrome oxidase activity in posterior colliculus and parietal cortex at 200, 

300, and 400 ppm COS after exposure for 3, 6, and 12 weeks.

Neurophysiology:

Changes in Brainstem Auditory Evoked Responses (BAERs) at 400 ppm after 2 weeks 

exposure. [Reference 1]

Changes in somatosensory evoked potentials (SEPs) at 400 ppm after 2 or 12 weeks. 

[Reference 2]

Changes in BAERs at 300 and 400 ppm COS for 12 weeks. NOAEL = 200 ppm. 

[Reference 2]

Behavior:

Changes in Functional Observational Battery (FOB) at 400 and 500 ppm with 2 week 

exposure [Reference 1], and motor activity and FOB altered by 400 ppm COS with 2 

week exposure. [Reference 2]

Some changes in FOB at 200, 300, and 400 ppm after 6 wks exposure, some of which 

lessened during continued exposures out to 12 wks. [Reference 1]

Summary

NOAEL of 200-300 ppm selected based on brain pathological lesions and 

neurophysiological alterations. 

Clear behavioral effects were observed at 400 ppm, and exposure to 200 or 300 ppm 

produced mild, and in some cases transient, changes.

Biochemistry changes were observed in brains of rats at all dose levels including 200 

ppm, the lowest dose evaluated. A NOAEL was not determined for biochemical changes.

References:

1. Morgan, D.L., Little, P.B., Herr, D.W., Moser, V.C., Collins, B., Herbert, R., Johnson, G.A., 

Maronpot, R.R., Harry, G.J., and Sills, R.C.: Neurotoxicity of carbonyl sulfide in F344 rats 

following inhalation exposure for up to 12 weeks. Toxicol. Appl. Pharmacol. 200(2):131-

145, 2004.

2. Herr, D.W., Graff, J.E., Moser, V.C., Crofton, K.M., Little, P.B., Morgan, D.L., and Sills, 

R.C.: Inhalational exposure to carbonyl sulfide produced altered brainstem auditory and 

somatosensory-evoked potentials in Fischer 344N rats. Toxicol. Sci. 95(1):118-135, 2007.

3. Sills, R.C., Morgan, D.L., Herr, D.W., Little, P.B., George, N.M., Ton, T.V., Love, N.E., 

Maronpot, R.R., and Johnson, G.A.: Contribution of magnetic resonance microscopy in the 

12-week neurotoxicity evaluation of carbonyl sulfide in Fischer 344 rats. Toxicol. Pathol. 

32:501-510, 2004.

Other Studies:

Sills, R.C., Harry, G.J., Valentine, W.M., and Morgan, D.L.: Interdisciplinary neurotoxicity 

inhalation studies: Carbon disulfide and carbonyl sulfide research in F344 rats. Toxicol. Appl. 

Pharmacol. 207(Suppl 2):S245-S250, 2005.