What percentage of radon is dangerous?

What is radon?

Radon is a radioactive gas that has no smell, colour or taste. Radon is produced from the natural radioactive decay of uranium, which is found in all rocks and soils. Radon can also be found in water.

Radon escapes from the ground into the air, where it decays and produces further radioactive particles. As we breathe, these particles are deposited on the cells lining the airways, where they can damage DNA and potentially cause lung cancer.

Outdoors, radon quickly dilutes to very low concentrations and is generally not a problem. The average outdoor radon level1 varies from 5 Bq/m3 to 15 Bq/m3. However, radon concentrations are higher indoors and in areas with minimal ventilation, with highest levels found in places like mines, caves and water treatment facilities. In buildings such as homes, schools, offices, radon levels can vary substantially from 10 Bq/m3 to more than 10 000 Bq/m3.  Given the properties of radon, occupants of such buildings could unknowingly be living or working in very high radon levels. 

Health effects of radon

Radon is a major cause of lung cancer. It is estimated that radon causes between 3% to 14% of all lung cancers in a country, depending on the national average radon level and the smoking prevalence.

An increased rate of lung cancer was first seen in uranium miners exposed to very high concentrations of radon. In addition, studies in Europe, North America and China have confirmed that even low concentrations of radon – such as those commonly found in residential settings – also pose health risks and contribute to the occurrence of lung cancers worldwide.

The risk of lung cancer increases by about 16% per 100 Bq/m3 increase in long time average radon concentration. The dose-response relation is assumed to be 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. In fact, smokers are estimated to be 25 times more at risk from radon than non-smokers. No other cancer risks or other health effects have been established to date, although inhaled radon can deliver radiation to other organs, but at a much lower level than to the lungs.

Radon in buildings

For most people, the greatest exposure to radon occurs in the home where people spend much of their time, though indoor workplaces may also be a source of exposure. The concentration of radon in buildings depends on:

• the local geology, for example the uranium content and permeability of the underlying rocks and soils;
• the routes available for the passage of radon from the soil into the building;
• the radon exhalation from building materials; and
• the rate of exchange between indoor and outdoor air, which depends on the construction of the building, the ventilation habits of the occupants, and the air-tightness of the building.

Radon enters buildings through cracks in the floors or at floor-wall junctions, gaps around pipes or cables, small pores in hollow-block walls, cavity walls, or sumps or drains. Radon levels are usually higher in basements, cellars and living spaces in contact with the ground.  However, considerable radon concentration can also be found above the ground floor.

Radon concentrations vary considerably between adjacent buildings, as well as within a building from day to day and from hour to hour. Because of these fluctuations, it is preferable to estimate the annual mean concentration of radon in indoor air by measurements for at least 3 months. Residential radon levels can be measured in an inexpensive and simple manner by means of small passive detectors. Measurements need to be based on national protocols to ensure consistency as well as reliability for decision-making. Short-term radon tests, done in compliance with national protocols, can be valuable when making decisions during time-sensitive situations, such as home sales or to test the effectiveness of radon mitigation work.

Reducing radon in indoor settings

Well-tested, durable and cost-efficient methods exist for preventing radon in new buildings and reducing radon in existing dwellings. Radon prevention should be considered when new structures are built, particularly in radon-prone areas. In many countries of Europe, and in the United States of America and China, the inclusion of protective measures in new buildings are included in building codes.

Some common ways of reducing radon levels in existing buildings include:

• increasing under-floor ventilation;
• installing a radon sump system in the basement or under a solid floor;
• avoiding the passage of radon from the basement into living spaces;
• sealing floors and walls; and
• improving the ventilation of the building, especially in the context of energy conservation.

Passive systems of mitigation can reduce indoor radon levels by more than 50%. When radon ventilation fans are added radon, levels can be reduced even 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 higher concentrations of radon than surface water from reservoirs, rivers or lakes.

To date, epidemiological studies have not confirmed an association between consumption of drinking-water containing radon and an increased risk of stomach cancer. Radon dissolved in drinking-water is released into indoor air. Normally, a higher radon dose is received from inhaling radon compared with ingestion.

The "WHO guidelines for drinking water quality" [1] (2011) recommend that screening levels for radon in drinking-water be set based on the national reference level for radon in air. In circumstances where high radon concentrations might be expected in drinking-water, it is prudent to measure radon concentrations. Straightforward and effective techniques exist to reduce the concentration of radon in drinking-water supplies by aeration or using granular activated carbon filters. Further guidance is available in “Management of Radioactivity in Drinking-water” [2] (2018).

WHO response

Indoor radon is a preventable risk factor that can be handled through effective national policies and regulations. The "WHO handbook on indoor radon: A public health perspective" [3] provides policy options for reducing health risks from residential radon exposure through:

• providing information on levels of radon indoors and the associated health risks;
• implementing a national radon programme aimed at reducing both the overall population risk and the individual risk for people living with high radon concentrations;
• establishing a national annual average residential radon concentration reference level of 100 Bq/m3, but if this level cannot be reached under the prevailing country-specific conditions, the reference level should not exceed 300 Bq/m3;
• developing radon measurement protocols to help ensure quality and consistency in radon testing;
• implementing radon prevention in building codes to reduce radon levels in buildings under construction, and radon programmes to ensure that the levels are below national reference levels;
• promoting education for building professionals and providing financial support to remove radon from existing buildings; and
• considering the inclusion of radon as a risk factor in national strategies related to cancer control, tobacco control, indoor air quality and energy conservation.

These recommendations are consistent with the International Basic Safety Standards [4] (2014), co-sponsored by WHO and other international organizations. WHO promotes the implementation of these radon standards, which ultimately support the 2030 Agenda for Sustainable Development Goals (SDG), and Target 3.4 on noncommunicable diseases. To help monitor national radon policies and regulations around the world, WHO has assembled a radon database [5] as part of the WHO Global Health Observatory.

Footnotes

1 Radioactivity is measured in units called Becquerels (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 meter of air (Bq/m3).

Radon is a naturally occurring radioactive gas that can affect properties of all types, ages, locations and uses. The gas is formed when uranium in the soil and rocks beneath us decays. When it permeates the ground into open air, it is quickly diluted to low concentrations, however if it rises into a building, it can become trapped and build to dangerous concentrations.

What risks are associated with radon exposure?

When concentration of the gas is high, the radioactive decay products are inhaled and some are deposited in the lungs, where they continue to emit radiation. Each year in the UK over 2000 people die from lung cancer, developed as a direct result of exposure to radon. The gas accounts for the second greatest number of lung cancer cases in the UK, second only to smoking.

Where is radon found?

Buildings with basements are also more susceptible to high levels of radon accumulating, as there is a larger surface area in contact with the soil through which the gas can permeate. Public Health England advise that any property with a basement, regardless of whether it is located in an Affected Area or not, will have an increased probability of containing high radon concentrations.

It is important to recognise that the maps are only an indicative guide as to which parts of the country are most likely to be affected by high levels of radon. They are not definitive. Properties in lower risk areas (white parts of the map) may still be affected by radon. The maps are not property specific and they do not indicate how high the radon concentrations are; they are based upon estimated number of properties with radon concentrations above the ‘action level’ rather than the radon concentrations likely to be found.

How do I know if there is radon in a building?

Radon is odourless, colourless and tasteless. To assess the level of radon in an existing building, a test must be conducted.

Various digital monitors are now available, however the most common method of radon testing involves placing special passive detectors in the property for a period of time before returning them to a laboratory for analysis. Air enters the detectors and any radon particles will make small etches on a special plastic lens within the detector. The shape, size and number of these etches is then analysed under a special microscope in the laboratory.

Passive radon detectors are small and discreet when in use, and the whole process including laboratory analysis is inexpensive.

As radon levels fluctuate according to seasonal and occupational variances (e.g. amount of ventilation through opening windows), a three month period is normally recommended to take such inconsistencies into account.

The result is given in a unit called becquerels and expressed as becquerels per cubic metre of air (Bq/m3).

The number of detectors required depends upon the size, layout and usage of the building. A radon testing provider can advise on this.

What is a high radon level and what are “Action Levels”?

Outside in the open air, average radon levels are approxmately 4 Bq/m3 across the UK. The average radon level inside UK homes is 20 Bq/m3. There is no known “safe” level of radon, but the lower the concentration you are exposed to, the lower the risk to your health.

Current advice from Public Health England is that “for levels below 100 Bq/m3, your individual risk remains relatively low and not a cause for concern. However, the risk increases as the radon level increases.”

The Government advise that remedial action should be taken in any residential property with an average annual radon level of 200 Bq/m3 or more. This is referred to as the Action Level.

In 2010, the Health Protection Agency (now part of Public Health England) began advising that households with radon levels over 100 Bq/m3 should “strongly consider” taking remedial action. It is also advised that when carrying our remedial action, the aim should be to reduce levels to below 100 Bq/m3 where possible. This is therefore referred to as the Target Level.

Different ‘action levels’ levels apply to workplaces – see our Information for Employers page for more details on dealing with radon in the workplace.

How does radon get inside a building?

Some radon will passively infiltrate into the building, for example through cracks in the foundations and gaps around service pipes. Evidence has also been found to demonstrate that radon can also pass through certain materials, even those which provide an adequate barrier to water penetration.

The main mechanism through which radon enters a property, however, is advection. This is the movement of the gas from the soil to the lowest point of pressure, which is usually inside the building. This means that the gas is literally being sucked from the ground into the building, and the greater this pressure difference is, the faster the rate at which the gas is drawn inside is.

How can high radon levels be reduced?

The good news is that there are several techniques available that can successfully reduce radon concentrations within a building, and prevent future intrusion of radon.

As radon enters a building due to the pressure differences between the building and the ground, the main methods of dealing with radon in existing buildings is to alter this pressure ratio. This can be done either through the installation of a positive pressure unit or a retrofit sump system.

Positive Pressure

Retrofit Sump System