"Effect of Radon on Health" has about 110 searches per month. The site ranks around 49th. The phrase "radon effect on health" has a similar volume, around 53rd place.
What the Gas Does
Radon decays in the air. The radioactive dust particles produced settle in the lungs when inhaled. The risk increases with concentration and the number of years spent in the home. Tobacco use further increases this risk. Health Canada lists it among the causes of lung cancer, after tobacco.
No Symptoms Indicate the Level
No smell, no headache that says "300 Bq/m³." The article on symptoms details this. Measurement is done with a 91-day test (74.72 $ taxes included). Above 200 Bq/m³, radon entry is reduced. Air is not treated with a fragrance or by leaving a window open permanently.