There is no defined acceptable level of hexavalent chromium emissions. The acceptable health risk for a given area is subject to many factors such as wind direction, wind speed, distance to the closest sensitive receptors (schools, residents, day care facilities, medical facilities), other toxic chemical compounds (TACs) emitting from the same facility, how many other facilities are proximate to the given area, and other factors. The determination of an acceptable level of hexavalent chromium in the specific case of Anaplex and Aerocraft, determined that 1 ng/m3 is good enough based on their own health risk assessment (HRA).
On a more general basis, in terms of concentration and sustained exposure CalEPA’s Office of Environmental Health Hazard Assessment (OEHHA) has calculated a cancer risk associated with exposure to Cr6 (hexavalent chromium) if that exposure continues for an entire lifetime. Continual exposure to 0.045 nanograms per cubic meter (ng/m3) of Cr6 from all sources combined for 30 years could increase cancer risk to 25 in a million. Exposure over shorter periods of time would be associated with much lower cancer risks. OEHHA has also developed a chronic Reference Exposure Level (REL) for Cr6. A chronic REL is a health-based benchmark that is set at a level at or below which adverse noncancer health effects are unlikely to occur in the general human population when exposed continuously over a lifetime. Levels above the REL do not indicate the health effects will occur, but rather, that the chances of these health effects occurring increase at levels above the REL. Non-cancer health effects associated with Cr6 include nasal, throat, or respiratory irritation or allergies. The chronic REL for Cr6 is 200 ng/m3 in air (0.2 μg/m3). Please see a fact sheet by OEHHA dated November 9, 2016 for more details.