Drs. Douglas Dockery at the Harvard School of Public Health and C. Arden Pope III at Brigham Young University speculated that exposure to PM might lead to a transient drop in blood oxygenation, which might have serious consequences in humans with heart or lung problems. The investigators designed a study to increase the possibility of observing PM effects by testing a potentially at-risk group (the elderly) at a time of year that historically had experienced relatively high levels of PM (the winter). They conducted the study in residents of the Utah Valley, because at this altitude (4,000 feet above sea level) greater declines in oxygen saturation would be expected than at sea level. Twice a day subjects used a small medical device called an oximeter to measure their blood oxygen saturation and pulse rate. One group of subjects also kept a diary of their clinical symptoms. Information about daily PM level was collected from fixed, outdoor monitors in the Utah Valley. Other weather variables, such as temperature and barometric pressure, were measured at a nearby weather station.