Even if the risk was linear/exponential with threshold at 12 for everyone, including infants and elderly, the 30% average reduction is likely mostly done by smoothing the peaks (since you probably have a base level of PM2.5). So you would have more than a 30% reduction in days in which you are exposed to above-threshold PM2.5

Btw, it's a very legitimate remark, please don't down vote the parent. (Sorry about meta commentary I try to avoid)