Radon accounts for about half the natural radiation we're exposed to in our lives and doctors say exposure to high levels can lead to cancer.

Radon gas is produced during the breakdown of radioactive elements in rock - in particular, volcanic rock, which is what most of Hong Kong is built on. Throughout the city, foundations were dug into the granite and there are basements into which ground water and gas from that granite seeps.

Radon is soluble in water, and underground streams can carry it long distances. It can seep into homes through cracks and holes in the foundations.

Adding to the problem, the concrete used in Hong Kong also emits radon gas, although safer forms have recently been developed. To make matters worse, developers build small apartments with low ceilings - the kind of places where you're likely to keep the air conditioning on throughout the year.

A survey in the early 1990s compared radon gas levels in buildings that were airconditioned and those using windows for ventilation. The levels were much higher in air-conditioned areas.

How serious is the problem? The WHO says radon may be partly to blame for a rise in cancer rates in urban societies.

The cancer you're most likely to suffer as a result of living and working in buildings with high levels of radon gas is lung cancer. Exposure to smoke or dust further increases your chance of getting lung cancer from radon gas.

So-called radon daughters hitch a ride on smoke and dust particles and bury themselves deep in your lungs, where they emit radiation.

Smoking has been banned from offices in Hong Kong and authorities hope to get it out of restaurants, shops and bars. But Hong Kong has a high rate of lung cancer among non-smokers, and a reason may be our heavily polluted indoor air.

There's been a big rise in breast cancer cases, and although there's no evidence that smoking is linked to this, indoor pollution in crowded spaces could be contributing. So take the WHO seriously. Open your windows and ban smoking from your home.