Indoor air pollution is a chimney stack and a smoggy city, but what it does not make sense to science and policy

Air pollution can be seen as a chimney stack and a smoggy city. But this can be a misleading picture. Indoor air pollution killed more than 3 million people in 2020, almost as many as did its outdoor counterpart. It has been largely invisible to science and policy.

Chemicals can irritate the lungs and eyes when used in building materials and fabrics. Volatile organic compounds can be found in paints, carpets and wood treatments. Persistent chemicals such as brominated fire retardants are embedded in modern furniture. These chemicals can react to make secondary pollutants, such as formaldehyde and PM 2.5, that have inflammatory and dangerous properties.

Poorlyventilated buildings are where moulds thrive. Increased severity of asthma is one of the health impacts that can be attributed to inhaling airborne mold. A two-year-old boy died of black mould in the rented flat he lived in, and a UK coroner ruled that it was due to exposure to black mould.

Such diversity makes it challenging to define what good indoor air quality looks like. CO2 can be used as a proxy. The metric works well for measuring emissions linked to people and respiration, but does not give a good idea about the prevalence of volatiles from solvent and mold.

Models of indoor air chemistry are needed to evaluate the rates at which pollutants are removed or form. Some models have been adapted from outdoor atmospheric-chemistry mechanisms, to account for reduction in light and ultraviolet levels and to estimate loss of pollution from indoor surfaces such as furnishings9. But the processes by which chemicals degrade indoors differ from those for outdoors. Oxidation, the loss of electrons by a molecule, can transform benign indoor chemicals like methane into harmful compounds like formaldehyde and secondary PM 2.5. Nitrogen dioxide, ozone and chlorine are more involved in oxidation than the main outdoor oxidant at low UV levels.

For moulds, bioaerosols and CO2, good building-management practices that ensure adequate heating, airflow and air exchange with outdoors are effective14. Increasing ventilation will create energy trade-offs, because heat is lost in colder weather, but engineered solutions are feasible.

The interventions need to be fully characterized. Multiple actions will be necessary in order to address the subset of effects that each solution addresses. As with most public-health interventions, many incremental improvements leading to wholesale change are more likely to be effective than is waiting for a small number of transformational improvements.

It is essential that decarbonization, building improvement and gains in indoor air quality are, as much as possible, delivered equitably across society. Those who can afford to pay will benefit the most from certain strategies, such as installing heat pumps and air filters.