The Government today issued a "smog alert" for the Easter weekend for ozone and polluting particles known as PM10s, which can affect people's health.
Officials at the Department for Environment, Food and Rural Affairs (Defra) said high levels of pollution are expected across England and Wales over the weekend because of warm and still conditions brought on by the high pressure system.
Some people, including those who have asthma, could be affected by the high pollution levels and may notice an impact on their breathing.
The public is being advised to take "sensible precautions", including avoiding taking exercise outside in the afternoon if they are susceptible to the pollutants and not taking unnecessary short car journeys - in a bid to help reduce the build-up of ozone.
Defra said elevated levels of PM10s and ozone, reaching high or moderate measures, were expected from now until at least Sunday, while other pollutants will remain at low levels.
Experts said people suffering from lung diseases including asthma, particularly the elderly, should be aware that their breathing could worsen.
Ground level ozone is formed when sunlight acts on nitrogen dioxides and other atmospheric substances which come from a range of sources, including petrol and other fuels.
Particulates, or PM10s, also come from sources such as vehicles.
The smog alert, the first for more than a year, comes as monitoring in London revealed that one site, on the Marylebone Road, has exceeded EU rules for the number of days in the year in which high levels of PM10s are permitted.
The UK will not face fines for the breach, as the EU has given Britain an extension which means it has until June before it has to start meeting the standards in the capital.
But Green Party London Assembly member Jenny Jones said London's residents and visitors should be made aware of the high pollution levels ahead of the Royal wedding.
Cher Piddock, Asthma UK Adviceline nurse, said the smog warning was a "timely reminder" that the combination of warm weather and pollution could pose health risks, and asthma symptoms could worsen during hotter weather due to higher levels of pollen and pollution.
She said: "Around two-thirds of people with asthma say pollution triggers their condition, so Asthma UK recommends that people who have pollution as a trigger avoid going out if air quality is poor.
"We also recommend that people always carry their inhaler, avoid exercising outdoors on hot days, especially in the afternoon, and keep windows shut whenever possible.
"It may also be helpful to keep a regular check on air quality levels in the local area, so that it is possible to make changes to plans for the following day if pollution levels will be high."