Multiple air quality measurement strategies exist due to different pollutants, emission sources, and modeling procedures. The most commonly measured air pollutants are particulate matter (PM), ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), volatile organic compounds (VOC), lead (Pb), and polycyclic aromatic hydrocarbons (PAHs). They are frequently measured using monitors, but monitoring is often sporadic, making it difficult to estimate the true level of exposure.[2-4]
Monitors only measure the air quality at a specific location. They capture point-source pollution well but may not capture the air quality a few miles away or accurately measure nonpoint-source pollution. Point-sources include stationary buildings such as industrial factories. Nonpoint-sources (e.g. motor-vehicle exhaust) have the potential to change location, and may differ based on time of day. For example, air pollutant levels may be higher during rush hour, which is difficult to capture using passive or active monitoring.
In order to estimate county or state-level air pollution, modeling techniques or proxy clinical data are utilized. Models such as the Community Multi-Scale Air Quality Model (CMAQ) extrapolate data from monitors, weather patterns, and vehicle emissions to provide large-scale geographic estimates. Usually, threshold levels are used to measure air pollution. Common measures are the number of days the Air Quality Index exceeds 100 or the number of days PM2.5 is measured to be higher than 12.0 micrograms per cubic meter (µg/m3). The Air Quality Index is updated daily by the EPA, and provides information at the zip code level. Other proxy air pollution measures include the number of emergency room visits for respiratory diseases such as asthma or the number of diseases associated with air pollution.
Water quality measures that are generated from actual water quality samples are very reliable. However, collecting water quality samples for all possible contaminants in a large geographic region is not feasible for a statewide or national study. National measures have included measuring the number or percentage of people on public water supplies. Others have used databases to determine specific violation counts for each water system. This has led to a national reporting system to the EPA. The EPA requires states to report when water systems reported Maximum Contaminant Levels higher than the acceptable limit. They maintain a database called SDWIS (Safe Drinking Water Information System), going back to 1993, which houses these violations. The database includes the specific contaminant, date of contamination, length of contamination, and other valuable pieces of information.
 Centers for Disease Control and Prevention. National Environmental Public Health Tracking Network. Accessed March 13, 2014.
 Centers for Disease Control and Prevention. Outdoor Air: Health Impacts of Fine Particles in Air. Last reviewed December 17, 2013. Accessed March 4, 2014.
 Samet J, Dominici F, Curriero F, Coursac I, Zeger S. Fine particulate air pollution and mortality in 20 US cities, 1987-1994. N Engl J Med. 2000;343:1742-1749.
 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.
 Air Now: Local Air Quality Conditions. United States Environmental Protection Agency. Last updated May 6, 2013. Accessed March 13, 2014.