Dr. Brunekreef and his colleagues assessed the correlation between personal, indoor, and outdoor PM2.5 concentrations for elderly people with cardiovascular disease living in Amsterdam, The Netherlands, and Helsinki, Finland. Measurements were taken between November 1998 and June 1999 using fixed monitoring sites near the subjects' residences and inside their homes using the same type of monitor. Personal exposures were monitored with a different type of sampler that the subjects kept with them at all times. The subjects were 50-84 of age, did not smoke, and had been diagnosed with coronary artery disease or other cardiac disorders. Information about time spent indoors and outdoors and in different activities (eg, cleaning, cooking, sleeping) was recorded using questionnaires.