Issues that Must be Addressed for Risk Assessment of Mixed Exposures: The U.S. EPA Experience with Air Quality

Abstract

Humans are routinely exposed to a complex mixture of air pollutants in both their outdoor and indoor environments. The wide diversity of these exposure scenarios and potential for regional transport present the health scientist with a challenge of how to appropriately address complex air pollution mixtures. Similarly, regulators are faced with mixture issues ranging from exposures, to health outcomes, and to associated uncertainties that are in need of more definitive and strategic information to support informed decisions. This article provides a perspective of an empiricist on the background related to the issue of air pollution mixtures. Historic and current regulatory platforms for dealing with mixtures are described. Using the topic of particulate air pollution, general guidance through the nuances of potential interactions among PM constituents is provided, along with alterative approaches and examples, and how these support scientific and regulatory agendas. The impact of new cell and molecular technologies is inevitable, and we must be prepared to take advantage of these and other cross-cutting methods as they become available. Such innovative approaches hold the secret to high-throughput biologic dissection of component interactions and mixture profiles, which will aid in the assessment of risk.

The author thanks Drs. Deirdre Murphy (Office of Air Quality Planning and Standards) and Linda Birnbaum (Experimental Toxicology Division/ORD for their critical reviews of the manuscript.

The research and information described in this article has been reviewed by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and the policies of the agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.

Notes

*The national-scale assessment (www.epa.gov/ttn/atw/nata/) is one of the U.S. EPA's National Air Toxics Assessment activities (NATA). The assessment focused on 33 toxics, 32 of which are among the “urban air toxics” (identified as those hazardous air pollutants posing the greatest risk to public health in urban areas).

*The effect occurring at the lowest exposure, and on which the RfC is based, is termed the “critical effect” (U.S. EPA, 1994).