Ozone - that’s a good thing, right? Well, it depends. Ozone (O3) high up in the atmosphere is protective - which is why international action to counteract the hole in the ozone layer is often cited as a beacon of collective environmental action, primarily through the 1990s.
However, ozone at ground level - also known as tropospheric ozone - can be very harmful. It mainly affects the lungs and breathing, causing chest pain and constriction, coughing, irritation, inflammation and congestion. It can also worsen existing conditions such as asthma, emphysema and bronchitis.
Country life
So if it’s so bad, why don’t we talk about it much? Our Breathe London monitors such as that locally in Brixton don’t measure it. Typically ozone is worse in the countryside than in inner cities (a reminder that air pollution is not purely an urban issue). However, it can affect cities on hot days, leading to alerts such as this recent one just before Clean Air Day. This report from the UK Health Security Agency expresses concern that ozone levels are likely to increase in coming decades.
The way the World Health Organisation thinks about guidelines for exposure to ozone is somewhat different to what we are used to for nitrogen dioxide and fine particulate matter. For ozone they have a peak season limit of 60 micrograms per cubic metre and 100 over other 8 hour periods. The peak season is defined as the average of the daily maximum 8 hour mean ozone concentrations in the six consecutive months with the highest six month running-average ozone concentration. We thought about trying to simplify WHO’s language there but thought best to go verbatim!
Knock on wood
Why is ozone treated so differently to other pollutants? It’s classed as a secondary pollutant, as it’s caused by the reaction of sunlight with other pollutants such as nitrogen dioxide and volatile organic compounds (VOCs, which can come from trees, cleaning products and many other sources). So there can be an interesting situation where nitrogen dioxide is actually reducing while ozone is increasing. Our lungs probably don’t want to have to choose between the two!
Although we can’t report it on from our Breathe London node, we did cover elevated ozone levels when working with a group of doctors riding through France towards Geneva, to raise awareness of the health affects of air pollution. As they got deep into the French countryside, their ozone levels were getting above those WHO limits.
How to reduce ozone at ground level? Basically the same strategies as we take to reduce the primary pollutants that are a precursor to ozone. Reduce nitrogen dioxide from moving away from petrol and diesel cars, and reduce VOCs through better choices of cleaning products, paints and so on. Don’t give that pesky sunlight anything to interact with!
But can’t trees be part of the problem? They can emit VOCs when under stress. This report from Los Angeles has shown that overheated trees can contribute both ozone and particulate matter pollution in the city. Thoughtful choices of the right trees for the right circumstances seems to be the best answer. From O-zone to A-zone!
Interesting to read that ozone is higher in rural areas compared with cities. What are the reasons and what is the significance of the differences for rural and city dwellers, taking account of the proportions of people affected?