Yeah it depends what question you are trying to answer: 1. Does my air purifier filter out PM2.5? or 2. Dos lower PM2.5 make me sleep better? For #1 there i

Yeah it depends what question you are trying to answer: 1. Does my air purifier filter out PM2.5? or 2. Dos lower PM2.5 make me sleep better? For #1 there is no placebo effect (there is no human element!)