To the Editor:
The National Morbidity, Mortality, and Air Pollution Study (NMMAPS) was designed to examine the health effects of air pollution in the United States. The primary question was whether particulate matter was responsible for the associations between air pollution and daily mortality. Secondary questions concerned measurement error in air pollution and mortality displacement.1 Since then, NMMAPS has been used to answer many important questions in environmental epidemiology.
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The data from 108 US cities for the years 1987–2000 comprise daily observations of mortality counts in 3 age groups, air pollutants (including particulate matter and ozone), and weather (including temperature).
The data were made publicly available first via the Web (http://www.ihapss.jhsph.edu/data/data.htm), and then via R.2 They were removed from public availability in 2011 due to privacy concerns.
We used a systematic review to find peer-reviewed papers or reports that used the NMMAPS data. There were 67 papers or reports in total (see the eAppendix [http://links.lww.com/EDE/A575] for the methods and a complete list). Thirty-three publications (49%) were based on the publicly available data (Table). The most common application was methodology (33 publications). The first authors came from 5 countries.
Sixty-seven publications are a substantial research output from one study, reflecting the originality and scope of the data. The data have been used to answer questions on the health effects of air pollution and temperature in the United States, and to answer methodological questions. Such applications also create benefits outside the United States, as new and refined methods can be used in other countries.
NMMAPS has been used to examine deaths during heat waves and to predict future heat-wave deaths due to climate change. Examining climate change was not an original goal of NMMAPS, but these data (which cover a wide range of climates over a long time) are an excellent resource for investigating this critically important question.
Research from NMMAPS has been used by US Environmental Protection Agency for regulatory impact statements on particulates3 and ozone, and by reports on climate change.4 It has influenced policy outside the United States, being cited in a UK government report on particulate matter5 and a World Health Organization report on air pollution.6
The easy availability of NMMAPS meant that researchers could write papers without the time and costs of data collection and ethics applications. The study has benefited the research careers of statisticians and epidemiologists around the world. This creates a virtuous circle, as researchers are more likely to stay in the field, produce more research outputs, and win grants. If this pattern is to continue, more researchers need to make their data publicly available.
US investigators who worked to make NMMAPS publicly available should be applauded. More formal rewards for providing access to data are difficult, which may be one reason why more data sets are not made freely available.7
A key obstacle to data-sharing is the ethics of sharing medical data, with the major concern being whether risks outweigh benefits. The number and influence of publications from NMMAPS outweigh (in our opinion) data-security concerns, particularly as the NMMAPS health data are aggregated and anonymized. Nonetheless, data-security concerns caused these data to be removed from public availability in 2011—a backward step for reproducible research.8 Thanks to NMMAPS, the United States has an improved understanding of the health effects of air pollution and heat waves. Furthermore, the utility of these data is not diminishing—9 papers were published in 2010, 7 in 2011, and additional papers are currently under review.
Adrian G. Barnett
Institute of Health and Biomedical
Innovation and School of Public Health
Queensland University of Technology
1. Samet JM, Zeger SL, Dominici F, et al.. National morbidity, mortality, and air pollution study. Part II: morbidity and mortality from air pollution in the United States. Health effects institute report #: 094-II. 2000. Available at: http://pubs.healtheffects.org/view.php?id=118
2. Peng RD, Welty LJ. The NMMAPS data package. R News.2004; 4: 10–14.
3. United States Environmental Protection Agency. Regulatory impact analysis for the federal implementation plans to reduce interstate transport of fine particulate matter and ozone in 27 states; correction of SIP approvals for 22 states. US EPA Office of Air and Radiation, 2011. Available at http://www.epa.gov/airtransport/pdfs/FinalRIA.pdf
4. Committee on the Effect of Climate Change on Indoor Air Quality and Public Health. Climate change, the indoor environment, and health. Institute of Medicine. The National Academies Press, 2011. Available at http://www.nap.edu/openbook.php?record
7. Pisani E, AbouZahr C. Sharing health data: good intentions are not enough. Bull World Health Organ.2010;88: 462–466.
8. Peng RD, Dominici F, Zeger S. Reproducible epidemiologic research. Am J Epidemiol.2006;163: 783–789.
Thanks to the Department of Biostatistics at the Johns Hopkins Bloomberg School of Public Health and the Health Effects Institute for making the National Morbidity and Mortality Air Pollution Study data publicly available. Thanks to Francesca Dominici and Michelle Bell for discussions and guidance.