How much serious of the indoor PM2.5 air pollution

Particle counter help us to check air quality, especially the PM2.5 fine particulate matters.
Because people stay indoor time is longer, from the perspective of the average daily cumulative level,
indoor inhalation of PM2.5 is 4 times that of outdoor inhalation.
At the same time, the indoor PM2.5 concentration and the outdoor concentration ratio is 0.67:1.
So we need the laser particle counter to check the air quality and then we can know how to change it.