The report examines changes in behavior, expenditures, and willingness to pay (WTP) as related to asthma severity. It is based on information for a panel of 82 asthmatics, representing a population expected to be sensitive to ambient oxidant levels. The study focuses on measuring mitigating behavior and its effect upon epidemiology and economic studies, and on the importance in morbidity valuation studies of using a WTP benefit measure rather than a cost of illness (COI) measure based only on work loss and medical expenditures. Most of the information was collected in two survey instruments: a daily diary completed for one month, and a general questionnaire. The daily diary's objectives were to determine the accuracy of perceptions about air pollution, whether these perceptions altered expectations about whether a bad asthma day (as defined by the respondent on a symptoms rating scale) would occur, and whether respondents altered their activities on bad air pollution days. The general questionnaire addressed how well COI measures estimate the most appropriate WTP measures for changes in illness.