The government has issued a smog warning, predicting high ozone levels across London, East Anglia, south-east and central England until Saturday.
Even healthy people can experience tightness of breath in smog
But what exactly is smog and why does it happen?
Q: What is summer smog?
Summer smog is the noxious air produced when sunlight acts on polluting substances released into the lower atmosphere.
Cars pump out nitrogen oxides and hydrocarbons. Industrial processes will also churn out a range of volatile compounds.
The photochemical reactions involving these "precursors", as they are known, will create harmful ozone.
Summer smog can also contain high levels of breathable dust, often referred to as particulate matter.
The summer months are prone to smog development because this is the time of year when high pressure systems bring prolonged and strong sunshine. High pressure systems are slow moving, and the pollution can get caught in their sinking air.
Q: But isn't ozone supposed to be a good thing?
Yes it is - high in the stratosphere, more than 10 miles (16km) up. It filters out harmful ultraviolet light. All life on Earth depends on this protective layer - but at ground level, ozone is bad thing.
Q: What can it do?
Ozone can irritate the lungs' airways, exacerbating the symptoms of those suffering from asthma and lung diseases.
But even normally fit and health people may experience a shortness of breath, some coughing, chest tightness, or just irritation of the nose and throat.
Q: What else can it do?
It impacts on plants, too. It hampers their ability to produce and store food, and that opens them up to disease, insects, and other pollutants. Prolonged summer smogs will damage the leaves of trees and even restrict crop production.
Q: Is it just an urban problem?
No. Many urban areas will have high levels of "bad" ozone, but rural areas can also be subjected to increased ozone levels because the wind carries ozone and the pollutants that form it hundreds of miles away from their original sources.
Q: What is the best advice?
Because ozone forms in hot weather, anyone who spends time outdoors in the summer may be affected, particularly children, outdoor workers and people exercising.
People who are concerned about their health should contact their general practitioner. Individuals suffering from lung diseases, including asthma, particularly if elderly, should be aware their symptoms might worsen.
They may need to consider modifying their treatment as they usually do when symptoms increase, consulting their doctor if this is not effective.
Q: What can be done to ease the problem?
The Department for the Environment has recommended some simple ideas.
It suggests trying to avoid using your car for short journeys - 1.5 miles (2.5km) or less.
If possible, do not use your car at all during periods of high pollution, and take public transport instead.
If you must drive, start your engine only once you are ready to move off, and do not rev the engine unnecessarily.
Avoid using products that might add to the problem - oil-based solvents, paints, glues, and varnishes. Use water-based ones if you can. And do not light bonfires.