This paper assesses whether responses to information about risk impact estimates of the relationship between ozone and asthma in Southern California. Using a regression discontinuity design, I find smog alerts significantly reduce daily attendance at two major outdoor facilities. Using daily time-series regression models that include year-month and small area fixed effects, I find estimates of the effect of ozone for children and the elderly that include information are significantly larger than estimates that do not. These results are consistent with the hypothesis that individuals take substantial action to reduce exposure to risk; estimates ignoring these actions are severely biased. (Contains 7 tables, 4 figures and 32 footnotes.)