These cases can be due to unsafe indoor air quality.
Moreover, there have been some health problems reported that are allegedly caused by ozone generator due to its toxic gas. It uses an electricity to convert oxygen into ozone. The more time the air passes through HEPA filters, the cleaner and safer the air becomes. This benchmark is said to be efficiently functional which can trap 99.97% of airborne particles larger than 0.3 microns. One micron is equivalent to 1/25,400 of an inch.