HEPA filters are just for particials, not fumes or vapors. So you won't be safe from everything. Wrong filters on a blower, can cause a increase in contamination INSIDE. Also most plans i have seen, just look too crappy for mine tast. Many have very little information about making those improvsed blowers and there filter connection vapor and fume tight. Which can cause leaks.

Also have you done a risk assesment? A terrorist release or a accidential release are always on certain places, terrorists want soft targets, with lots of victims. So they target big city's, public transport, etc. Accidents with chemicals plants are transport are only a problem when living close to a plant or hazardous materials route.

Most people are beter of in investing there money in a safer car, which will more likely safe there life's.