"Air Toxics" Reaching North America Lower than Previously Thought

February 8, 2006

A new study on the concentration of hazardous air pollutants (or "air toxics") reaching North America, published in the Journal of the Air and Waste Managements Association, Jan. 2006, shows that the background concentrations of 18 hazardous air pollutants are as much as 85% lower than reported in previous studies. This in turn suggests that regional and local contributions to most urban air toxics concentrations in the U.S. are much higher than previously thought.


Historically, measurements of air toxics have focused on urban areas. As a result, air toxics concentrations in background air, i.e., air upwind of urban areas in North America, have been poorly characterized. Such information is critical for assessing the influence of local emission controls on local pollution, since reductions in concentrations are limited by background concentrations. In a collaborative effort between scientists from Sonoma Technology, Inc., under contract from the Environmental Protection Agency, and the NOAA Earth System Research Laboratory, background concentrations of 18, high priority air toxics were determined on the basis of atmospheric observations from two different atmospheric monitoring networks. These concentrations were compared to those typically observed in urban air and also to "one-in-a-million" cancer benchmark concentrations.


For most air toxics, the background concentrations were 6-32% of the concentrations typical for urban air. An exception was carbon tetrachloride where background concentrations of this air pollutant in background air were 99% of those typically measured in urban air. Health risks posed by hazardous air pollutants are often gauged by cancer benchmark concentrations that, given exposure over a 70-yr lifetime, will result in a one-in-a-million chance of developing cancer. The authors found that on average the concentrations for benzene, chloroform, carbon tetrachloride, formaldehyde, and chromium fine particulate were higher in U.S. urban areas than the "one-in-a-million" cancer benchmark concentrations. Additional emissions of these gases in urban areas would substantially increase the risk associated with these air toxics.

Added value was brought to this research by making use of measurements from NOAA's North American Carbon Atmospheric Observing System (ACOS) currently being built and operated by NOAA ESRL scientists. Because the CAOS is aimed at obtaining remote or regional information, it is also ideal for providing frequent, high-quality, calibrated background measurements upwind, over and downwind of North America.

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