Preliminary epidemiologic investigation of the relationship between the presence of ambient hazardous air pollutants (HAPs) and cancer incidence in Harris County.

Executive Summary

Title:  A preliminary investigation of the association between hazardous air pollutants and lymphohematopoietic cancer risk among residents of Harris County Texas

Authors: Kristina M. Walker, Ann L. Coker, Elaine Symanski, Philip J. Lupo

Affiliation:  University of Texas Health Science at Houston, School of Public Health

In the recent months, there has been a great deal of interest surrounding Houston’s air quality and the potential for high levels of pollutants in the ambient environment to adversely affect citizens’ health.  Historically, attention has focused on criteria pollutants such as ozone.  However, there is increasing concern regarding another group of pollutants, known as hazardous air pollutants (HAPs).  HAPs are a class of 189 chemicals which are known or suspected to have adverse effects on health.1  Unlike criteria pollutants, there are no national standards regulating acceptable levels of these compounds in the ambient environment.  Two HAPs, benzene and 1,3-butadiene, may be of particular concern in Houston due to the large volume of emissions as well as their potential to cause cancer in humans.

While recent reports have commented on the elevated ambient levels of hazardous air pollutants (HAPs) in certain areas of Houston, relative to other cities, few studies have assessed the health effects of HAPs for Houstonians and none have evaluated the association between ambient levels of these pollutants and lymphohematopoietic cancer risk in this population.  Accordingly, we conducted a population based analysis of ambient environmental levels of HAPs and leukemia and lymphoma incidence in Harris County.  We identified all cancer cases, including adult and childhood cases, diagnosed and reported to the Texas Cancer Registry from 1995-2003.  We then used existing air monitoring data collected from 1992-2003 by the Texas Commission on Environmental Quality (TCEQ) to estimate ambient census tract levels to benzene and 1,3-butadiene.  Additionally, we estimated risk of developing leukemia and lymphoma associated with residential proximity to the Houston Ship Channel (HSC) as another surrogate of HAP exposure.  We assigned cancer cases to a particular census tract based on their residence at diagnosis as reported to the TCR.  We then calculated cancer rates separately for adult and childhood cancers for each census tract.  We further accounted for gender, age, socio-economic status, and ethnicity. 

We found a 56% increased risk of acute lymphocytic leukemia among children living within two miles of the HSC (p-value=0.01) compared with children living more than 10 miles from the HSC.  We found no increased risks of developing any other childhood leukemia or lymphoma associated with living within two miles of the HSC.  We did, however, find that compared with children living in areas with the lowest estimated 1,3-butadiene levels estimated from monitoring data collected by the Texas Commission on Environmental Quality (TCEQ), children living in areas with the highest levels had a 40% (p-value=0.02), 38% (p-value=0.05) and 153% (p-value=0.03) increased risk of developing any type of leukemia, acute lymphocytic leukemia and acute myeloid leukemia, respectively.  Higher 1,3-butadiene levels were not associated with childhood lymphoma or with adult leukemia or lymphoma.  Higher estimated ambient benzene levels based on TCEQ monitoring data were not associated with childhood leukemia or lymphoma rates. 

At the suggestion of several environmental scientists, we repeated our analyses for childhood leukemia using the United States Environmental Protective Agency’s 1999 National Air Toxics Assessment (NATA) modeled ambient 1,3-butadiene and benzene levels.  In general, we saw a similar pattern to that observed using the TCEQ monitoring data.  When comparing childhood leukemia rates in those census tracts with the highest 1,3-butadiene levels with those in the lowest, a 32% increase in cancer risk was noted (p=0.09).  This compares with a 40% increase (p=0.02) in risk for all types of leukemia in children based on the TCEQ monitoring data estimates for 1,3 butadiene.

Among adults, neither proximity to the Houston Ship Channel, nor levels of benzene or 1,3-butadiene was consistently associated with leukemia or lymphoma risk.  Additional analytic studies with more refined exposure assessment methods are planned.  However, observing a specific health effect of HAPs in light of recently documented elevated levels of two known carcinogens, benzene and 1,3 butadiene, in Houston,2 strongly suggests a need to explore this issue further and possibly take action to limit potential exposure to HAPs in Houston.

1. About Air Toxics, Health and Ecological Effects. 2006. Retrieved On: 4-2-06. http://www.epa.gov/air/toxicair/newtoxics.html
2. A closer look at air pollution in Houston.  Identifying priority health risks: Report of the Mayor's task force on the health effects of air pollution. Houston, Texas: University of Texas, Health Science Center. Institute for Health Policy.