National Hospital Discharge Survey and Air Quality Data from the U.S. Environmental Protection Agency

1999-2005 NHDS Pollution Exposure Data File

The National Center for Health Statistics (NCHS) has conducted the National Hospital Discharge Survey annually since 1965. The NHDS is a national probability survey designed to provide information on characteristics of inpatients discharged from non-Federal short-stay hospitals in the United States. Each year the NHDS collects data from a sample of approximately 300,000 inpatient records acquired from a national sample of about 500 hospitals. More information about the survey is available at the National Hospital Discharge Survey website.

NCHS has created data files with air pollution information, available from the Environmental Protection Agency, specifically designed to be linked to NHDS data files. The NHDS Pollution Exposure data files contain the NHDS admission dates and inpatient zip-code of residence with corresponding air pollution exposures. These air pollution exposures can be linked to in-house NHDS files by zip-code and admission date to obtain an analytic file with both air pollution exposures and discharge information. Because residential zip-code and admission date are in-house variables, the NHDS Pollution Exposure data files and the in-house NHDS data files are available through the NCHS Research Data Center (RDC). Information about the RDC is available at the Research Data Center website.

Pollution measurements at air monitoring locations were obtained from the EPA. See Air Quality System (AQS) Data for Downloading to access that data.. For each discharge, pollution exposure measurements for seven days and six weeks prior to the date of admission were averaged from monitors within 20 and 5 miles of the geo-coded zip-code center. Exposures for carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), ozone (O3) were averaged using distance weighting, where measurements from monitors closer to the zip-code center contributed more than those farther away. Averages for particles (PM2.5 and PM10) were calculated without distance weights because particulate matter is more homogeneously distributed. Because not all NHDS inpatients live near monitors, most discharge records will have missing values for one or more pollutants after linkage. More records have values for exposures calculated using 20-mile radii than using 5-mile radii.