For me, outdoor air quality is just one piece of the equation. So many things we bring into our homes "shed" dust, odors, chemicals that I feel confined indoor air may be riskier than outdoor air. In addition, if I run them full time, I actually see a marled difference in the amount of dust on my floors and furniture. Not sure of your electric rates, but ours cost less than $5.00/mo each to run full time.
Seems like a big return for a liitle money.