I wear my mask when out on my ebike to protect myself from exhaust fumes and chemical sprays on residential lawns. In the warmer months I need to wear it when travelling outside of town in a car to protect myself from agricultural pesticides drift as I live in an area that grows mostly corn and beans, probably mostly GMO and sprayed liberally with pesticides. I also wear it in grocery stores, thrift stores and the mall. Even with the mask I rarely venture into a mall.
For more about masks, see