The World Health Organization publishes a set of indoor air quality guidelines covering specific chemical pollutants, household fuel combustion, dampness and mold, and radon. These guidelines establish concentration limits or risk-based targets for pollutants commonly found inside homes, schools, and workplaces, giving governments and building professionals a science-based benchmark for protecting public health.
Three core WHO publications form the framework: guidelines for selected chemical pollutants (2010), guidelines for dampness and mold (2009), and guidelines for household fuel combustion (2014). A separate handbook addresses indoor radon. Together, they cover the pollutants most likely to reach harmful levels indoors.
Pollutants the Guidelines Cover
The WHO selected nine chemical substances for its indoor air quality guidelines: benzene, carbon monoxide, formaldehyde, naphthalene, nitrogen dioxide, polycyclic aromatic hydrocarbons (using benzo[a]pyrene as a marker), radon, trichloroethylene, and tetrachloroethylene. Each was chosen because it has known indoor sources, poses documented health risks, and is frequently found indoors at concentrations high enough to cause harm.
Some of these pollutants get firm numerical limits. Others, particularly cancer-causing chemicals with no safe threshold, are addressed through risk estimates that describe how much your cancer risk rises per unit of exposure. The practical message for those substances: reduce exposure as far as possible.
Formaldehyde
Formaldehyde is one of the most common indoor pollutants. It off-gases from pressed-wood furniture, flooring, paints, and some cleaning products. The WHO sets a short-term guideline of 0.1 mg/m³, measured over a 30-minute period. This threshold is designed to prevent eye and airway irritation in the general population, including sensitive individuals.
The lowest concentration reported to cause eye irritation in studies is 0.36 mg/m³ over four hours, with visible signs like increased blinking and redness appearing at 0.6 mg/m³. The WHO guideline sits well below both of those levels. Importantly, the 0.1 mg/m³ short-term limit also protects against long-term effects, including cancer. Separate modeling of long-term risks produced a comparable value of about 0.2 mg/m³, so meeting the stricter short-term target covers both concerns.
Benzene
Benzene is classified as a cancer-causing substance in humans, and the WHO states plainly that no safe level of exposure can be recommended. Instead of a fixed limit, the guidelines provide a risk ladder. At an air concentration of 1.7 μg/m³, you face roughly a 1-in-100,000 excess lifetime risk of leukemia. At 17 μg/m³, that risk rises to 1 in 10,000.
Indoor benzene comes from tobacco smoke, attached garages, stored solvents, and certain building materials. Because there is no safe floor, the WHO’s practical recommendation is to reduce indoor levels as low as possible. This is one area where ventilation and source removal matter more than hitting a specific number.
Nitrogen Dioxide
Gas stoves, unvented heaters, and vehicle exhaust seeping in from garages are the main indoor sources of nitrogen dioxide. The WHO guidelines set two limits: 200 μg/m³ as a one-hour average, and 40 μg/m³ as an annual average. These mirror the organization’s outdoor air quality guidelines, reflecting the fact that nitrogen dioxide behaves the same way in your lungs regardless of where you breathe it.
Short-term spikes above 200 μg/m³ can trigger airway inflammation, particularly in people with asthma. Chronic exposure above the annual limit is linked to reduced lung function and increased respiratory infections in children.
Naphthalene
Naphthalene is the chemical behind the sharp smell of mothballs, and it also enters indoor air from certain fuels, tobacco smoke, and some consumer products. The WHO guideline for naphthalene is 0.01 mg/m³, applied as an annual average. This value was derived from animal studies showing nasal tissue damage, with large safety factors built in to account for differences between species and variation among individuals.
Radon
Radon is a naturally occurring radioactive gas that seeps into buildings from the ground. It is the second leading cause of lung cancer after smoking, and the risk increases proportionally with exposure: there is no threshold below which radon is considered completely safe.
The WHO recommends a reference level of 100 Bq/m³ (becquerels per cubic meter) to minimize health hazards. Where country-specific conditions make that target impractical, the chosen reference level should not exceed 300 Bq/m³, which corresponds to roughly 10 millisieverts of radiation per year. Testing is the only way to know your home’s radon level, since the gas is colorless and odorless. Mitigation typically involves sealing foundation cracks and improving sub-floor ventilation.
Household Fuel Combustion
A separate set of WHO guidelines targets emissions from cookstoves and household heaters, a major health concern in low- and middle-income countries where billions of people burn solid fuels indoors. Rather than setting ambient concentration limits, these guidelines use emission rate targets for the stoves themselves.
For fine particulate matter (PM2.5), stoves without a chimney or flue should emit no more than 0.23 mg per minute. Vented stoves, which channel smoke outside, are allowed up to 0.80 mg per minute. Carbon monoxide targets follow the same logic: no more than 0.16 grams per minute for unvented devices and 0.59 grams per minute for vented ones. Meeting these targets requires either cleaner-burning fuels (such as gas or ethanol) or well-designed improved cookstoves.
Fine Particulate Matter (PM2.5)
The 2021 WHO Global Air Quality Guidelines tightened the annual PM2.5 recommendation from 10 μg/m³ down to 5 μg/m³. While these guidelines were developed primarily for ambient (outdoor) air, the WHO has presented them as relevant to both indoor and outdoor environments. Indoor sources of fine particles include cooking, candles, incense, tobacco smoke, and infiltration of outdoor pollution.
The 5 μg/m³ target is aggressive, and many urban environments struggle to meet it even outdoors. For indoor spaces, achieving it generally requires a combination of source control (minimizing combustion indoors), mechanical ventilation with filtration, and standalone air purifiers in high-pollution areas.
Dampness and Mold
Unlike the chemical pollutant guidelines, the WHO’s dampness and mold guidelines are qualitative rather than numerical. No specific spore count or moisture threshold is given as a safe limit. Instead, the core recommendation is straightforward: prevent or minimize persistent dampness and visible microbial growth on interior surfaces and within building structures.
This approach reflects the complexity of mold exposure. Hundreds of species can grow indoors, each producing different allergens and irritants, making a single concentration limit impractical. The health effects of living in damp or moldy housing are well documented and include worsening asthma, respiratory infections, and allergic symptoms. The WHO’s position is that any visible mold or persistent moisture problem warrants remediation, regardless of the species involved.
How Countries Use These Guidelines
WHO guidelines are not legally binding. They serve as evidence-based recommendations that individual countries can adopt, adapt, or use as benchmarks when writing their own regulations. Some nations incorporate the WHO values directly into building codes or indoor air standards. Others use them as aspirational targets while setting less stringent enforceable limits based on local conditions.
For individuals, the guidelines offer a practical framework for evaluating your own indoor environment. If your home has a gas stove in a poorly ventilated kitchen, nitrogen dioxide is worth thinking about. If you have new furniture or recent renovations, formaldehyde levels may be elevated for weeks or months. If you live in a region with granite bedrock, radon testing is a sensible step. The WHO guidelines give you the numbers to put those risks in context.