It occurs due to rise in global warming which occurs due to increase in temperature of atmosphere by burning of fossil fuels and release of harmful gases by industries. (hubpages.com)
You should be aware that air cleaners generally don't remove everything — most don't remove bad smells or remove all potentially harmful gases from the air — be suspicious of such claims. (treehugger.com)
Paper is burned at the end of its lifecycle and this process uses electricity, creates waste and emits harmful gas. (frontporchblog.com)