Indoor air quality: a survey of available data sets and recommendations for future research at the U.S. Department of Air Science and Environment (ASAS)
Observations against which to benchmark models are also scant for indoor air. It is difficult to access private spaces to gather data. Experiments studying indoor air have historically been low on funders’ priority lists. But research has picked up in the past five years; some data sets have been collected that cover a wide range of chemicals indoors, although only for a handful of test homes, mostly in the United States. Such data sets are key for testing whether assumptions about what controls air indoors are correct and whether interventions are likely to be effective11. They need to be expanded to include all types of building and construction.
Building materials, fabrics and furniture also give off chemicals that can irritate the lungs and eyes. Household products that have Volatile Organic compounds in them include paints, carpets and wood treatments. Persistent chemicals such as brominated fire retardants are embedded in modern furniture. These chemicals can react to produce secondary pollutants, such as formaldehyde and PM2.5, that have inflammatory and carcinogenic properties.
There are poorly ventilated buildings that produce moulds. There can be effects on health from inhaling mold. For example, in November 2022, a UK coroner ruled that the death of a two-year-old boy had been caused by prolonged exposure to black mould in the rented flat he lived in.
Some of these parameters can be easily measured, such as the size of a room, outdoor concentration and the rate at which air is exchanged. Others, for example the strengths of sources and sinks, are much harder to assess. The grams of NOx emissions per vehicle kilometres driven are some of the types of pollutants quantified. These are compiled into emissions inventories, some being used for regulatory purposes, others for research. However, few such estimates are available for indoor air.
To evaluate the rates of pollutants being removed from the indoor environment, models of indoor air chemistry are needed. 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 one or more electrons by a molecule) is important, and can transform relatively benign indoor chemicals such as methane into harmful compounds, including formaldehyde and secondary PM2.5 (ref. 10). Ozone, chlorine and nitrates are the main oxidizers at low indoor UV levels.
Heat exchangers offer one solution, by recapturing energy in a fluid before air leaves the house. These are not easy to install in older buildings. The effects of various technologies designed mainly for decarbonization need to be quantified.
Ventilation and UV innovations need to be characterized. Each solution will address only a subset of effects, and multiple actions will be needed. 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.
decarbonization, building improvement and gains in indoor air quality must all be delivered equally across society. Householders investing in air filters and heat pump installation will skew the benefits for those who can’t afford them.