The County Health Rankings show us that where we live matters to our health. The health of a community depends on many different factors - ranging from health behaviors, education and jobs, to quality of health care, to the environment.

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Environmental Quality

What Is It?

Adequate environmental quality in terms of good air and water quality are prerequisites for good health. Poor air or water quality can be particularly detrimental to the very young, the old, and those with chronic health conditions. Our measures of environmental quality represent the population health risks from ambient air pollution. Population-based and cohort studies have demonstrated that several pollutants, notably ozone and fine particulate matter (particulates less than 2.5 micrometers in diameter), can contribute to increased morbidity and mortality.[1-4] The County Health Rankings represent the health risks of ambient air pollution based on these two commonly measured pollutants. Our measures include the estimated number of days that a county’s air quality is unhealthy for sensitive populations due to either one of these pollutants.

Why Do We Measure It?

The relationship between elevated air pollution and compromised health has been well documented.[5] The negative consequences of ambient air pollution include decreased lung function, chronic bronchitis, asthma, and other adverse pulmonary effects.[1-3] Exposure to excess levels of ozone or fine particulate matter are correlated with an increase in hospital emergency room visits and hospitalizations among asthmatics and others with compromised respiratory function.[2] Increases in these pollutants are associated with greater risk of death due to cardiopulmonary and cardiovascular conditions and ischemic heart disease.[3,6] All-cause mortality also is associated with greater concentrations of ozone and fine particulate matter.[3,6,7] 

Because of the strong relationship between air pollution and health, the Clean Air Act of 1990 requires the United States Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six criteria pollutants, which include carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide. The standards established for these pollutants are the levels at which the contaminant is considered to compromise the public’s health. Special attention is given to “sensitive” populations, including children, seniors, and individuals with respiratory or cardiopulmonary conditions.[8] The clear impact air quality has on human health and the importance the EPA places on monitoring select pollutants shows that the Air Quality focus area is a critical one to include among the Rankings’ environmental measures.

Measurement Strategies

Several measures can be used to represent air quality. The most common include annual average values for fine particulate matter and ozone. The measures may be used due to incomplete data coverage. Missing data is problematic itself, and the measures, by masking variation in these pollutants over time and geography, are also less than ideal. According to a study by Samet et al., only half of the 19 cities included had ozone data for all the days in the study, and fine particulate matter (FPM) data was collected much less frequently. For half of the cities, FPM data was collected on only 20% (or less) of the days in the study. [7] Though ozone samples are collected more frequently than fine particulate matter samples, ozone levels tend to be more heterogeneous within a testing area, making it difficult to estimate the true level of exposure.[3,7,9] Geographic coverage becomes a greater problem in rural communities, many of which do not have air quality monitors. In 2007, only 29 of Wisconsin’s 72 counties had air quality monitors.[10] Highly refined versions of these measures can be calculated using the Community Multi-Scale Air Quality Model (CMAQ), an air quality simulation model that uses weather and emissions data to estimate chemical concentrations in the air. However, CMAQ estimates are usually available for areas of 12 square kilometers (7.5 square miles), and air monitor data are only available for one-third of all days in the year. CMAQ estimates tend to be somewhat biased, often underestimating pollutant concentrations.[11]

Two other measures, prepared by the EPA and the National Air Toxics Assessment (NATA), include a measure of cancer risk (cases per 1,000,000) and the respiratory hazard index. To determine the cancer and non-cancer respiratory hazard risk due to air pollutants, NATA models exposure data for 33 air toxics considered most harmful to human health. The cancer risk measure estimates lifetime cancer risk attributable to these air toxics given a lifetime (70 years) of continuous exposure and is reported as the incidence of cancer per 1,000,000 people. The respiratory hazard index measures the lifetime risk of non-cancer respiratory conditions also based on modeling of emissions data. If the hazard index is equal to or less than 1, no adverse health effects are expected. A hazard index that exceeds 1 suggests a greater risk of respiratory conditions due to exposure to air pollutants. Though available for most counties, NATA data are only released every three years. The most current NATA assessments, made available in June 2009, use data from 2002.[12]

What Is the County Health Rankings Measurement Strategy?

In the County Health Rankings, we use two measures to represent environmental quality: annual number of days that air quality was unhealthy for sensitive populations due to (1) fine particulate matter and (2) ozone concentrations.

Because of the problems inherent in measuring air quality already mentioned, particularly the lack of air quality monitors, the Public Health Air Surveillance Evaluation (PHASE) project team, which includes researchers from the Centers for Disease Control and Prevention (CDC) and the EPA, developed an innovative strategy to estimate air quality. The EPA’s first step to address insufficient data collection sites was the creation of an air quality simulation model, the Community Multi-Scale Air Quality Model (CMAQ).[13] To improve upon CMAQ, the PHASE project uses both CMAQ output and monitor data in a hierarchical spatial-temporal model to estimate daily ozone and fine particulate matter concentrations throughout the year.[11]

Researchers used the model output to estimate peak fine particulate matter and ozone concentrations for each day in the year and, using National Ambient Air Quality Standards, estimate the number of days that the air quality was poor for sensitive populations due to these contaminants.

Measure Strengths & Limitations

The advantages of the estimates presented in the County Health Rankings include:

  • The ability to estimate air quality for smaller geographic areas and shorter time frames, such as a 24-hour period. CMAQ estimates are usually available for areas of 12 square kilometers (7.5 square miles), and air monitor data are only available for one-third of all days in the year.
  • The ability to control for bias in CMAQ estimates, which tend to underestimate contaminant concentrations in the air. By using air monitor data, researchers can ensure that the PHASE model output better represents the true concentration of ozone and fine particulate matter in the air.

As a result, the County Health Rankings is able to present air quality estimates for all counties in the continental United States.[11]

The PHASE project estimates for the annual number of days that air quality was unhealthy for sensitive populations due to fine particulate matter and ozone concentrations have some disadvantages. The estimates, though more current than NATA data, are based on data from 2005. Additionally, unlike NATA, the Rankings only offer data on two contaminants. Though fine particulate matter and ozone are commonly viewed as two of the pollutants most harmful to health, the PHASE estimates do not provide any measure of cancer risk or respiratory hazard due to other criteria contaminants. Finally, data are only available for the contiguous United States, providing no estimates of air quality for Alaska and Hawaii.

 

References

[1] Bascom R, Bromberg PA, Costa DA, et al. Health effects of outdoor air pollution. Am J Respir Crit Care Med. 1996;153:3-50.
[2] Bell ML, McDermott A, Zeger SL, Samet JM, Dominici F. Ozone and short-term mortality in 95 US urban communities, 1987-2000. JAMA. 2004;292:2372-2378.
[3] Jerrett M, Burnett RT, Pope CA, et al. Long-term ozone exposure and mortality. N Engl J Med. 2009;360:1085-1095.
[4] Pope CA, Ezzati M, Dockery DW. Fine-particulate air pollution and life expectancy in the United States. N Engl J Med. 2009;360:376-386.
[5] Katsouyanni K, Touloumi G, Spix C, et al. Short term effects of ambient sulphur dioxide and particulate matter on mortality in 12 European cities: Results from time series data from the APHEA project. Br Med J. 1997;314:1658-1663.
[6] Dominici F, McDermott A, Daniels M, Zeger SL, Samet JM. Revised analyses of the National Morbidity, Mortality, and Air Pollution Study: Mortality among residents of 90 cities. J Toxicol Environ Health A. 2005;68:1071-1092.
[7] Samet JM, Dominici F, Curriero FC, Coursac I, Zeger SL. Fine particulate air pollution and mortality in 20 US cities, 1987-1994. N Engl J Med. 2000;343:1742-1749.
[8] Air and radiation: National Ambient Air Quality Standards (NAAQS). United States Environmental Protection Agency Web Site. www.epa.gov/air/criteria.html. Updated January 17, 2010. Accessed January 17, 2010.
[9] Smith RL, Xu BW, Switzer P. Reassessing the relationship between ozone and short-term mortality in US urban communities. Inhal Toxicol. Sep 2009;21:37-61.
[10] Athens J, Bekkedal M, Malecki K, Anderson H, Remington PL. Measuring the environmental health of Wisconsin's counties. Wis Med J. 2008;107:169-175.
[11] McMillan NJ, Holland DM, Morara M, Feng J. Combining numerical model output and particulate data using Bayesian space-time modeling. Environmetrics. 2009;21:48-65.
[12] National Air Toxics Assessments. United States Environmental Protection Agency Web Site. www.epa.gov/ttn/atw/natamain/. Updated August 19, 2009. Accessed January 17, 2010.
[13] Boothe V, Dimmick F, Talbot TO. Relating air quality and environmental public health tracking data. In: Environmental Exposure and Health, proceedings of the First International Conference on Environmental Exposure and Health; Oct. 5-7, 2005; Atlanta, GA.