WHO: Radon and cancer

Fact sheet N°291
Updated September 2009


Key facts

  • Radon is the second most important cause of lung cancer in many countries.
  • Radon is estimated to cause between 3% and 14% of all lung cancers, depending on the average radon level in a country.
  • Radon is much more likely to cause lung cancer in people who smoke, and is the primary cause of lung cancer among non-smokers.
  • Radon-induced lung cancers are mainly caused by low and moderate rather than by high radon concentrations, because of the large number of people exposed to indoor radon in homes with such low concentrations.
  • The lower the radon concentration in a home, the lower the risk as there is no known threshold below which radon exposures carries no risk.

What is radon?

Radon is a chemically inert, naturally occurring radioactive gas. It has no smell, colour or taste. Radon is produced from the natural radioactive decay of uranium, which is found in rocks and soil. Radon can also be found in water.

Radon escapes easily from the ground into the air, where it disintegrates through short-lived decay products called radon progeny. As radon progeny decay, they emit radioactive alpha particles and attach to aerosols, dust and other particles in the air. As we breathe, radon progeny are deposited on the cells lining the airways where the alpha particles can damage DNA and potentially cause lung cancer.

Outdoor radon levels are usually very low. The average outdoor radon level varies between 5 and 15 Bq/m3 [Radon radioactivity is measured in Becquerel (Bq). One Becquerel corresponds to the transformation (disintegration) of one atomic nucleus per second. Radon concentration in air is measured by the number of transformations per second in a cubic metre of air (Bq/m3)]. Indoors, radon levels are higher, with highest levels found in places such as mines, caves and water treatment facilities.

Health effects of radon

In many countries, radon is the second most important cause of lung cancer after smoking. The proportion of lung cancers attributable to radon is estimated to range from 3 to 14%.

Significant health effects have been seen in uranium miners who are exposed to high levels of radon. However, studies in Europe, North America and China have confirmed that lower concentrations of radon – such as those found in homes – also confer health risks and contribute substantially to the occurrence of lung cancers worldwide [1, 2, 3].

The risk of lung cancer increases by 16% per 100 Bq/m3 increase in radon concentration. The dose-response relation is linear – i.e. the risk of lung cancer increases proportionally with increasing radon exposure. Radon is much more likely to cause lung cancer in people who smoke.

Radon in homes

For most people, the greatest exposure to radon comes from the home. The concentration of radon in a home depends on:

  • the amount of uranium in the underlying rocks and soils
  • the routes available for the passage of radon into the home
  • the rate of exchange between indoor and outdoor air, which depends on the construction of the house, the ventilation habits of the inhabitants, and the sealing of windows.

Radon enters homes through:

  • cracks at concrete floor-wall junctions
  • gaps in the floor
  • small pores in hollow-block walls
  • sumps and drains.

Radon levels are usually higher in basements, cellars or other structural areas in contact with soil.

Radon concentrations can vary between adjacent homes, and can vary within a home from day-to-day and from hour-to-hour. Because of these fluctuations, estimating the annual mean concentration of radon in indoor air requires measurements of mean radon concentrations for at least three months.

Most countries have adopted an indoor air radon concentration of 200–400 Bq/m3 as a reference level above which mitigation measures should be taken.

Reducing radon in homes

Radon levels in homes can be reduced by:

  • improving the ventilation of the house
  • avoiding the passage of radon from the basement into living rooms
  • increasing under-floor ventilation
  • installing a radon sump system in the basement
  • sealing floors and walls
  • installing a positive pressurization or ventilation system.

Radon safety should be considered when new houses are built, particularly in high radon areas. In Europe and the United States of America, the inclusion of protective measures in new buildings has become a routine measure. In some countries it has become a mandatory procedure. Passive systems of mitigation have been shown to be capable of reducing indoor radon levels by up to 50%. When radon ventilation fans are added radon levels can even be reduced further.

Radon in drinking water

In many countries, drinking water is obtained from groundwater sources such as springs, wells and boreholes. These sources of water normally have much higher concentrations of radon than surface water from rivers, lakes and streams.

In many countries, radon concentrations of 20 Bq/l – in some instances above 100 Bq/l – have been measured in individual water supplies. To date, epidemiological studies have not found an association between radon in drinking water and cancer of the digestive and other systems. The WHO guidelines for drinking water qualityrecommend repeated measurements to be implemented if radon in public drinking water supplies exceeds 100 Bq/l.

WHO response

WHO recommends that countries implement national programmes to reduce the population’s risk from exposure to the national average radon concentration, as well as reducing the risk for individuals exposed to high radon levels. Building codes should be implemented to reduce radon levels in homes under construction. A national reference level of 100 Bq/m3 is recommended. However, if this level cannot be reached under the prevailing country-specific conditions, the reference level should not exceed 300 Bq/m3.

WHO has established the International Radon Project (WHO-IRP) in which over 30 countries have formed a network of partners to identify and promote programmes that reduce the health impact of radon. WHO-IRP aims to:

  • estimate the global health impact of exposure to residential radon
  • identify effective strategies for reducing the health impact of radon
  • promote sound policy options for prevention and mitigation programmes
  • raise public and political awareness about the consequences of exposure to radon
  • evaluate and monitor mitigation measures to ensure their effectiveness.

In 2009, WHO-IRP published the WHO handbook on indoor radon, which provides recommendations and policy options for reducing health risks from residential radon exposure.

WHO also assists Member States in continuously estimating the number of lung cancers attributable to radon exposure, which will allow assessment and monitoring of health impacts through future radon prevention and mitigation activities.

  • Adjusting Lung Cancer Risks for Temporal and Spatial Variations in Radon Concentration in Dwellings in Gansu Province, China. Lubin JH et al; 2005; Radiat. Res; 163:571-579.
  • Residential Radon and Risk of Lung Cancer: A Combined Analysis of 7 North American Case-Control Studies. Krewski D et al; 2005; Epidemiology; 16:137-145.
  • Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. Darby S et al; 2005; BMJ; 330(7485):223-227.