Which gas is considered a known human lung carcinogen?

Radon is recognized as a known human lung carcinogen due to its radioactive properties and the way it affects human health when inhaled. It is a naturally occurring gas that can accumulate in buildings, particularly in confined areas such as basements and lower levels, when it seeps up from the ground. Radon decay products can attach to airborne particles; when inhaled, they can damage lung tissue and increase the risk of lung cancer over time. This association has been confirmed through various epidemiological studies, which have shown a direct link between radon exposure and lung cancer rates, making it a significant health concern.

Other gases listed, while they may have negative health effects, are not classified as known human lung carcinogens. Carbon dioxide is a natural product of respiration and does not have direct carcinogenic effects at normal levels. Ozone, while harmful at ground level as an air pollutant, is primarily an irritant to the respiratory system rather than a carcinogen. Ammonia is also an irritant and can cause respiratory issues but is not recognized as a carcinogen. Therefore, radon's classification as a lung carcinogen sets it apart in terms of its health risks.