U.S. Joint Fire Science Program


Date of this Version


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Published by Joint Fire Sciences Program


US government work.


Several state air quality agencies have developed policies to issue air quality health index (AQI) warnings based on low values of visual range (Vr). Vr has been defined in the context of how far away a black object has to be such that it is just noticeable or visible. This distance at which a landscape feature can just be detected is referred to as the Vr. AQI warnings are based on the levels of particulates (PM2.5) resulting from fire smoke, often with less than 24-hr average concentrations. Because monitoring data are not available in all places where an AQI warning might potentially be given, human-observed visual conditions (i.e., sighting distant targets to determine Vr) have been used to estimate ambient fine particulate (PM2.5) concentrations. This procedure, originally developed in the arid West, may be particularly questionable when applied where higher humidity (especially in the humid Southeast) interacts with background sulfate and nitrate particulates and other aerosols from non-fire sources to reduce visibility. Human errors estimating Vr can be large. One result may be that the public is given an incorrect impression of air quality risks to their health and well-being; either the AQI or other indices are overestimates, causing undue public alarm, or underestimates from which the public, or at least sensitive sections of the public, undergo avoidable risks.