London’s outdoor air quality is watched closely, from the London Air Quality Network’s reference stations to the Breathe London low-cost sensor grid mapping pollution street by street. But most exposure to airborne pollutants for most residents happens indoors — at home, at work, in transit — and that environment sits largely outside the monitoring evidence base built around ULEZ, boroughs and daily air quality indexes. Understanding indoor air pollution as its own respiratory risk, distinct from (but connected to) the outdoor picture, fills a real gap in how Londoners are told to protect their lungs.
This matters because the sources are different, the people most exposed are often the most vulnerable, and the tools used to measure it are not the same tools that produce London’s public air quality data.
What’s Actually in London’s Indoor Air
Indoor air pollution comes from a mix of combustion, materials and damp, layered on top of whatever drifts in from outside. Cooking is a leading source of fine particulate matter (PM2.5), nitrogen dioxide and carbon monoxide in an ordinary kitchen. Heating appliances, open fires and solid-fuel or wood-burning stoves add more of the same — and UK evidence has flagged that even stoves meeting current approval standards can produce sharp spikes in indoor particulate levels during use, not just from older or poorly maintained units. Tobacco smoke remains a well-established contributor where smoking happens indoors.
Beyond combustion, building materials, furnishings, paints and household products such as cleaning sprays and air fresheners release volatile organic compounds. Dampness and mould, exacerbated by poor ventilation, are a further significant source — and UK paediatric health bodies have pointed out that over three million families in the UK live in housing with problems like damp and inadequate ventilation, which compounds indoor air exposure for children specifically. Radon, a naturally occurring radioactive gas that seeps in from the ground, adds a long-term lung cancer risk in some properties, independent of anything happening outside.
Why Indoor Pollution Hits Children and Vulnerable Residents Hardest
The respiratory health effects tracked in UK evidence fall unevenly. In children, indoor pollutant exposure is linked to a higher risk of asthma and wheezing, exacerbation of existing asthma symptoms, impaired lung development, and a greater likelihood of respiratory infections. Maternal exposure to fine particulates has also been linked to lower birth weight and preterm birth. Children from more deprived backgrounds are more likely to face poor air quality both indoors and outdoors at once — the two exposures compounding rather than offsetting each other.
In adults, the same pollutant categories are associated with a higher risk of chronic obstructive pulmonary disease and lung cancer, particularly where wood or solid-fuel burning is a regular indoor source, alongside asthma, respiratory irritation, and knock-on effects including cardiovascular strain, headaches and reduced concentration. None of this shows up in a borough-level outdoor air quality reading, because it isn’t being generated outdoors.
Outdoor Monitoring vs Indoor Air Quality: What Each Actually Measures
London’s outdoor monitoring infrastructure and indoor air quality tracking are built for different jobs, use different equipment, and answer different questions. The table below sets out how they diverge.
| Dimension | Outdoor monitoring (e.g. London Air Quality Network) | Indoor air quality tracking |
|---|---|---|
| Primary purpose | City-wide policy, planning and public health reporting | Air quality in the specific room or home someone occupies |
| Typical pollutants tracked | PM2.5, PM10, NO2, SO2, ozone | PM2.5, VOCs, and CO2 as a ventilation proxy, plus temperature/humidity |
| Method | Reference-grade stations plus diffusion tubes and a low-cost community sensor network for street-level detail | Room-placed sensors, sensitive to source proximity and calibration |
| Reading frequency | Hourly or near-continuous at fixed sites | Continuous where deployed, but coverage is sparse and mostly voluntary |
| What it misses | Anything generated inside a building after outdoor air has infiltrated or been filtered | City-wide comparability; results depend heavily on individual household behaviour |
Where Indoor and Outdoor Pollution Meet
The two are not separate systems — they interact. Outdoor pollutants, particularly nitrogen dioxide and particulate matter from traffic, infiltrate indoor spaces through ventilation, gaps in the building envelope, and open windows, especially in dense urban housing. How much gets through depends heavily on filtration: the effectiveness of any mechanical ventilation or filter in a home materially changes how much of London’s outdoor air quality reading actually reaches the air someone breathes inside. Some pollutants behave differently once inside: ozone levels in particular drop substantially indoors, especially with windows closed, while others generated purely indoors (cooking particulates, VOCs from products, mould) have no outdoor equivalent to compare against at all.
That means a London postcode with a favourable outdoor air quality reading can still sit above a household with poor ventilation, an unflued heater or persistent damp, and vice versa. Treating the outdoor index as a full proxy for respiratory risk understates what’s actually happening for anyone spending most of their day inside — which, for most people, is most of the time.
What This Means for the Evidence Base
London’s outdoor air quality monitoring is genuinely comprehensive by international standards, and it remains the right tool for tracking traffic-related pollution, informing policy such as low-emission zones, and comparing boroughs over time. But it was never designed to capture cooking fumes, damp-driven mould spores, VOCs from household products or wood-burning stove spikes — the sources that make up a meaningful share of what residents actually inhale. A population health picture of respiratory risk in London that stops at the outdoor monitoring network is, by design, only measuring part of the exposure. Closing that gap means treating indoor air quality as its own evidence base, not an afterthought to the outdoor one, particularly for children and households already facing poor housing conditions where the two risks compound.