What a Heatwave Actually Does to the Air You Breathe

Most of us experience a heatwave as a temperature story. We check the forecast, note the record-breaking numbers, and make plans around staying cool. This week, temperatures across England hit 35.1°C in London, breaking records for the hottest May day ever recorded. The heat made headlines for good reason.

But there's a second story that plays out alongside every heatwave, one that's harder to see and rarely makes the front page. When temperatures climb like this, air quality changes too. Sometimes significantly.

Heat Doesn't Just Warm the Air. It Changes It.

During a heatwave, the same high-pressure system that brings clear skies and intense sunshine also slows down air movement. Wind speeds drop. The atmosphere becomes more stable, with warm air sitting over cities like a lid. Pollutants that would normally disperse get trapped close to the ground, where people actually breathe.

That means nitrogen dioxide from traffic, volatile organic compounds from vehicles and industry, and fine particulate matter from a range of sources all build up rather than clearing. The result is that even if emissions don't increase, the concentration of pollutants at street level can rise significantly during a prolonged hot spell.

Research from the National Centre for Atmospheric Science found this pattern clearly during the 2022 UK heatwave. Harmful pollutants rose sharply as stagnant air trapped them near the surface.

Ground-Level Ozone: The Invisible Side of Sunshine

The most important air quality shift during a heatwave involves ground-level ozone. Most people associate ozone with the protective layer high in the stratosphere. But at ground level, ozone is a pollutant, and a harmful one.

Here's how it forms: when sunlight hits nitrogen oxides (from vehicle exhausts) and volatile organic compounds (from fuel, solvents, and other sources), a chemical reaction produces ozone. The hotter and sunnier the day, the faster this reaction happens.

During the 2022 UK heatwave, monitoring stations along the South East coast recorded ozone concentrations that were nearly double the World Health Organisation's recommended exposure limits. This wasn't an anomaly. It's a pattern that repeats every time sustained heat arrives.

Ground-level ozone can irritate the lungs, worsen respiratory conditions, and cause discomfort even for people without pre-existing health issues. It's not something you can see or smell easily, which is part of what makes it a challenge. The air might look perfectly clear on the hottest day of the year and still carry elevated ozone levels.

What Happens Indoors

There's a common assumption that staying indoors during a heatwave solves the air quality problem. In reality, indoor air is closely connected to what's happening outside.

Outdoor pollutants, including ozone and fine particulate matter, enter buildings through open windows, ventilation systems, and gaps in the building envelope. During a heatwave, most people open windows wide to try to cool down, which can actually increase indoor pollutant levels during peak afternoon hours when ozone concentrations are at their highest.

At the same time, heat itself causes indoor materials to release more volatile organic compounds. Furniture, flooring, paints, and cleaning products all off-gas at higher rates when temperatures rise. This is happening precisely when many homes are less ventilated than usual, because people close up during the day and rely on fans rather than fresh airflow.

The result is a complex indoor air environment that can deteriorate during the exact conditions when people spend more time inside.

Why This Matters Beyond the Heatwave

Heatwaves are becoming more frequent and arriving earlier in the year. The fact that May 2026 broke records that stood since 2003 tells us something about the trajectory. And each time sustained heat arrives, it brings this secondary air quality impact with it.

The challenge is that most people have no way to know what's happening to the air around them in real time. Government monitoring stations provide regional estimates, but they can't tell you what's happening in your living room, your office, or your child's school during a specific afternoon.

This is where granular, local air quality data becomes genuinely useful. Understanding the relationship between temperature and air quality at the level of individual spaces, not just city-wide averages, helps people make better decisions. When to ventilate. When to keep windows closed. Whether a particular room is accumulating pollutants that need addressing.

PurerAir's sensors measure PM2.5, PM10, CO₂, and VOCs, exactly the pollutants that shift during events like this. Having that data available in real time turns an invisible problem into something you can actually respond to.

A Quieter Kind of Preparedness

We've become reasonably good at preparing for heat itself. We check forecasts, buy fans, stay hydrated. But preparing for what heat does to air quality is still unfamiliar territory for most people.

It doesn't require dramatic action. Simple awareness, like knowing that ozone peaks in the afternoon, that opening windows at the wrong time can make indoor air worse, or that building materials release more chemicals in the heat, can inform small but meaningful decisions.

As heatwaves become a more regular part of UK summers, understanding their impact on air quality isn't a niche concern. It's becoming a basic part of living well in a warming climate.

Related reading

• summer ozone and air quality

• why indoor air quality deteriorates

• WHO air quality guidelines

PurerAir builds air quality sensors and tools that help people understand the air in the spaces where they live and work. Our sensors measure PM2.5, PM10, CO₂, VOCs, and noise in real time, with transparent, verifiable data.