Risk from Airborne Gaseous and Particulate Air Toxics in Metropolitan Phoenix

Hilary Hafner (1) Theresa O'Brien (1) PETER HYDE (2)

(1) Sonoma Technology, Inc. (2) Arizona State University, Tempe

     Abstract Number: 83
     Last modified: October 26, 2009

Abstract
In metropolitan Phoenix, Arizona in 2005 the Joint Air Toxics Assessment Project, a consortium of tribal, local, state, and federal environmental officials, measured particulate and gaseous air toxics species. Subsequent numerical analyses show that these air toxics pose a human health risk of 600 – 1300 lifetime excess cancer cases per one million population. Annual averages of particulate species reveal that of the six with chronic health-based guidelines – arsenic, cadmium, diesel particulate matter (DPM), chromium VI, manganese, and nickel – the first three exceed guideline values, but the last three are within them at all urban sites. Expressed as excess lifetime cancer cases per 1,000,000 population, the risk from particulate air toxics is dominated by DPM, with arsenic and cadmium combined contributing the remaining 10%. Among the urban sites the various populations would be expected to develop 200 to 500 excess cancers per million people through a lifetime of exposure to these particulate air toxics. These figures become five times greater if all carbonaceous particulates, rather than just those from diesel combustion, are included. Depending on the particulate species urban concentrations are enriched from three to 40 times above background. Situated on the urban perimeter, the two tribal sites generally have lower concentrations than the urban core sites.

For the gaseous air toxics nine different compounds, including benzene, formaldehyde, and others, together pose a genuine human health risk, though not as severe as the particulates: excess lifetime cancer cases per 1,000,000 population ranged from about 120 to 180, depending on the urban monitoring site. Combining the risk from gaseous and particulate species gives a net excess cancer rate of about 300 – 700 cases (and from three to four times this rate if all carbonaceous particulates, rather than just DPM, are considered equally deleterious). Because exposure analyses were not part of the JATAP, these calculated risks, based only on the measured ambient concentrations rather than exposure concentrations, are twice as high as the actual risk, as other urban air toxics studies have shown that exposure concentrations are about one half of the ambient ones.

The realistic and best estimate of risk to human health from both gaseous and particulate air toxics in metropolitan Phoenix, expressed as excess lifetime cancer cases per 1,000,000 population, is

• from 75 to 350 cancer cases, from just the diesel exhaust, arsenic, cadmium, and the nine gaseous species, depending on the monitoring site and its neighborhood; and

• from 600 to 1,300 cancer cases, if organic carbonaceous particulates are included.