What is radon?

Radon is an invisible, odourless, radioactive gas. It is formed by the disintegration of radium, which is a decay product of uranium. Radon emits alpha particles and produces several solid radioactive products called radon daughters.

Some amounts of radon gas and radon daughters are present everywhere in the soil, water, and air. Particularly high radon levels occur in regions where the soil or rock is rich in uranium. Radon is emitted by radium in the ground, groundwater and building materials. It can enter the indoor air where it and its decay products accumulate in poorly ventilated areas. Harmful levels of radon and radon daughters can accumulate in confined air spaces, such as basements and crawl spaces.

Radon daughters are inhaled with air and deposit in the lungs. The lung absorbs alpha particles emitted by the radon daughters. The resulting radiation dose increases the risk of lung cancer.

What are the health effects of radon?

Inhaling radon daughters increases the risk of lung cancer. The link between the concentration of radon daughters in the air and the risk of lung cancer is based mainly on data from a study of lung cancer mortality among uranium miners and other workers exposed to very high levels of radon daughters. There is no direct evidence linking indoor radon exposure in buildings to an increased risk of lung cancer.

Radon gas is chemically unreactive. It does not react with body tissues. While some inhaled radon does dissolve in the body fluids, the resulting concentration is so low that the radiation dose from the radon gas itself is negligible.

Radon daughters are solid particles. Most of the radon daughters become attached to tiny dust particles (aerosols) in the indoor air. A variable proportion remains unattached. When these particles are inhaled, a fraction of both attached and unattached radon daughters is deposited in the lungs. Inside the lung, radon daughters emit alpha particles that are absorbed in the nearby lung tissues. Since alpha particles cannot penetrate more than a fraction of a millimeter into the tissue, the damage is confined to the lung tissue in the immediate area.

Radon daughters also emit some low intensity beta particles and gamma rays that travel farther through the lung tissue. Because beta and gamma rays are absorbed over a large lung volume (i.e., not concentrated in a small area), their harmful effects are thought to be minimal.

People exposed to high radon levels have an increased risk of developing lung cancer. The extent of risk depends on the radon concentration in the air and the duration of exposure. The Committee on the Biological Effects of Ionizing Radiation (BEIR, IV, 1988) has estimated lifetime risk of 350 extra lung cancer cases if one million people are exposed to 1 Working Level Month (WLM) of radon daughters. In a recent report (BEIR VI, 1999), the number of lung cancer cases due to radon exposure in homes in the United States has been estimated to range from about 3,000 to 32,000.

Smoking increases the risk of lung cancer. Smokers exposed to radon daughters are at greater risk of developing lung cancer.

How does radon enter buildings?

Radium in the soil directly under a building is normally the major source of indoor radon. Less important sources of radium are in ground water and building materials.

The presence of uranium in soil and rock is an important indicator of places where radium and radon can be present. Because radon is a gas, a fraction of the radon produced in the soil can find its way into a building. The rest is trapped in the soil. In the air, radon decays to radon daughters that are solids, and are present in the building air as fine particles.

The concentration of radon and radon daughters in the indoor air depends on:

· the amount of radium in the soil and

· the ease with which the radon it produces can move through soil and building walls where it can then mix with the room air.

Because radon is a gas, changes in the atmospheric pressure also affect its emission from the ground and its accumulation in the building air.

The concrete floor and walls in the basement slow down the movement of radon from the soil into the building. However, cracks in the floor, wall slab joints, and the drainage system allow radon to enter a building.

Indoor radon concentrations are almost always higher than outdoor concentrations. Once inside a building, the radon cannot easily escape. The sealing of buildings to conserve energy reduces the intake of outside air and worsens the situation. Radon levels are generally highest in cellars and basements because these areas are nearest to the source and are usually poorly ventilated.

What are acceptable levels of indoor radon?

In Canada, the Atomic Energy Control Board (AECB) sets radiation exposure limits. It gives two types of exposure limits--one for occupationally exposed persons and another for the general public. The annual occupational exposure limit is 4 WLM. The annual exposure limit for the general public is 70 Bq/m3. In homes and other non-occupational settings, the maximum permissible annual average concentration of radon daughters caused by the operation of a nuclear facility is 0.02 WL (radon level 148 Bq/m3). Health Canada recommends 0.1 WL (radon level 800 Bq/m3) as an upper limit. If this upper limit is exceeded, remedial action should be taken to reduce the radon levels. However, because there is some level of risk at any level, Health Canada suggests that homeowners may want to reduce levels of radon to as low as possible.