Which of the following is considered a major indoor air pollutant?

Radon is a major indoor air pollutant, a naturally radioactive gas that seeps into buildings from the ground and rock beneath.

Mercury is a toxic metal of concern in water and industry, not a typical indoor gaseous pollutant here.

Helium is an inert gas that poses no pollution hazard indoors.

Argon is also an inert gas and is not an indoor pollutant of concern.

Radon accumulates in poorly ventilated rooms and basements where it can build up.

Long-term exposure to indoor radon is a leading cause of lung cancer after smoking.