Emergency planning and the acute toxic potency of inhaled ammonia.

Ammonia is present in agriculture and commerce in many if not most communities. This report evaluates the toxic potency of ammonia, based on three types of data: anecdotal data, in some cases predating World War 1, reconstructions of contemporary industrial accidents, and animal bioassays. Standards and guidelines for human exposure have been driven largely by the anecdotal data, suggesting that ammonia at 5,000-10,000 parts per million, volume/volume (ppm-v), might be lethal within 5-10 min. However, contemporary accident reconstructions suggest that ammonia lethality requires higher concentrations. For example, 33,737 ppm-v was a 5-min zero-mortality value in a major ammonia release in 1973 in South Africa. Comparisons of secondary reports of ammonia lethality with original sources revealed discrepancies in contemporary sources, apparently resulting from failure to examine old documents or accurately translate foreign documents. The present investigation revealed that contemporary accident reconstructions yield ammonia lethality levels comparable to those in dozens of reports of animal bioassays, after adjustment of concentrations to human equivalent concentrations via U.S. Environmental Protection Agency (EPA) procedures. Ammonia levels potentially causing irreversible injury or impairing the ability of exposed people to escape from further exposure or from coincident perils similarly have been biased downwardly in contemporary sources. The EPA has identified ammonia as one of 366 extremely hazardous substances subject to community right-to-know provisions of the Superfund Act and emergency planning provisions of the Clean Air Act. The Clean Air Act defines emergency planning zones (EPZs) around industrial facilities exceeding a threshold quantity of ammonia on-site. This study suggests that EPZ areas around ammonia facilities can be reduced, thereby also reducing emergency planning costs, which will vary roughly with the EPZ radius squared.