Ischemic heart disease (IHD), also called coronary artery disease or coronary heart disease, is a condition that the blood vessels are narrowed or blocked due to the deposition of cholesterol plaques on their walls. This affects the supply of blood to the heart and reduces the supply of oxygen and nutrients to the heart musculature. This may eventually result in a portion of the heart being suddenly deprived of its blood supply leading to the death of that area of heart tissue, resulting in a heart attack. IHD, with its associated symptoms, presents a major public health challenge. It is well known that IHD risk increases with age, smoking, cholesterol level, blood pressure and the disease is more common in men, diabetes patients and those who have close relatives with ischemic heart disease. The part played by the external environment, however, still remains something of a mystery.
Numerous epidemiological studies have indicated that exposure to fine particulate matter (particles smaller than 2.5 micrometers, PM
2.5) is associated with asthma, respiratory infections, lung cancer, cardiovascular problems, and premature death [1–5]. A few have examined coronary heart disease, finding evidence for acute effects on mortality and hospital admissions [6–8]. Recently, attention has focused on whether there is an association between chronic exposure to air pollution and coronary heart disease . In an ecological study at the census enumeration district level, Maheswaran et al.  found an association between nitrogen oxides and, to a lesser extent, particulate matter (PM10) and carbon monoxide, and coronary heart disease mortality in Sheffield, UK.
Typically, air pollution epidemiological studies rely on ambient observations from sparse monitoring networks to provide metrics of exposure, as in studies of PM and cardiovascular diseases [2, 4, 9–12]. Methods of exposure assessment include averaging multiple monitors within each enumeration unit or study site [4, 10, 11], assigning the exposure value of the nearest monitor to each case/control [2, 12] and spatial interpolation/modelling method . Ground monitoring data often lacks spatially complete coverage. Assessment of the exposure to air pollution using in situ observations is hampered by the difficulties in the highly variable concentrations in space and time. Ground monitors are rare in rural areas and temporal air quality estimates for particulates can vary on an hourly to weekly basis. Public health concerns compel efforts to broaden spatial and temporal coverage. The repetitive and broad-area coverage of satellites may allow atmospheric remote sensing to offer a unique opportunity to monitor air quality at continental, national and regional scales. Studies have found correlations between satellite aerosol optical depth (AOD), which describes the mass of aerosols in an atmospheric column, and PM2.5 ground concentration measurements in the eastern and mid-west USA [13–17] and other land parts of the world [18–20]. Satellite AOD could be used as an alternative indicator of air quality in air pollution epidemiological studies.
The MODIS (Moderate Resolution Imaging Spectrometer) instrument flies on polar-orbiting and sun-synchronous Terra and Aqua satellites of the Earth Observation System (EOS). MODIS performs measurements in the solar to thermal infrared spectrum region from 0.410 to 14.235 μm. The MODIS sensor was expected to be the key for monitoring global aerosol properties. Not only have MODIS aerosol products been used to answer scientific questions about radiation and climate, they are being used for applications not previously intended. Some examples include monitoring surface air quality for health [15, 16, 21–24]. In our recent study [17, 25] using the year 2004 data, we spatio-temporally collated daily MODIS AOD and PM2.5 mass concentration monitored by US Environmental Protection Agency (EPA)'s Air Quality System (AQS). Pearson's correlation analysis and geographically weighted regression (GWR) were conducted. We found significant positive correlations between PM2.5 and AOD to the east of the -100° longitude line during warm months (April-September). Eighteen of the twenty sites with highest correlation values (r > 0.8, p = 0.05) are in the east. The average correlation is 0.67 in the east and 0.22 in the west. GWR predicts well in the eastern US and poorly in the west, as indicated by a map of local R square. The coefficient raster surface for AOD exhibits regional variation. The relationship between PM2.5 and AOD is not spatially consistent (stationary) across the conterminous states. Eastern US shows higher AOD coefficient values, while values in the west are lower. Since there exists correlation between PM2.5 and MODIS AOD in the eastern US, if we could make a hypothesis that PM2.5 has adverse effect on CIHD, then we could also hypothesize that there is an association between MODIS AOD and CIHD.
The objective of this study was to examine if there is an association between particulate air pollution and chronic ischemic heart disease (CIHD). The study adopted an ecological method using aggregate disease mortality data at the county level for the eastern United States and MODIS derived AOD data. Exploratory spatial analysis methods and regression models were used to link CIHD mortality rate with satellite AOD.