OSHA's former limit for phosphoric acid was 1 mg/m3 as an 8-hour TWA. The Agency proposed adding a 15-minute STEL of 3 mg/m3 based on the ACGIH recommendation, and NIOSH (Ex. 8-47, Table N1) concurred with this proposal. In the final rule, the Agency is establishing a 1-mg/m3 TWA and a 3-mg/m3 STEL for this substance. Phosphoric acid is a colorless, odorless solid at temperatures below 21 deg. C but becomes a viscous, clear liquid at higher temperatures.
In humans, there have been reports of respiratory irritation from exposure to phosphorus pentoxide fume at concentrations of between 3.6 and 11.3 mg/m3; concentrations of 100 mg/m3 were unendurable except to workers who had developed a tolerance to the fume over time (Rushing 1957, as cited in ACGIH 1986/Ex. 1-3, p. 483). The AIHA Hygiene Guide for phosphoric acid reports that this substance is less hazardous than either nitric or sulfuric acid (AIHA 1957/ Ex. 1-709).
To protect unacclimatized workers from the risk of respiratory irritation, OSHA proposed a TWA limit of 1 mg/m3, with a STEL of 3 mg/m3, for phosphoric acid. No comments, other than NIOSH's, were received on this proposal. The Agency concludes that the combined 8-hour TWA and STEL limits are necessary to reduce this significant risk of irritation, which is considered by OSHA to be material impairment of health and which has been shown to occur at levels only slightly above those permitted by the TWA alone. Therefore, OSHA is establishing a 15-minute STEL of 3 mg/m3 to supplement its 8-hour TWA PEL of 1-mg/m3 TWA PEL for phosphoric acid.