Health warnings have been issued as air pollution levels are set to become "very high" in parts of the UK in what is already set to be the warmest day of the year.
Children and the elderly and those with respiratory problems have been told to avoid strenuous activity as high levels of pollution and some Saharan dust are expected to meld with warm conditions.
The government has set its smog alert at the highest level, with warnings to avoid physical exertion given to all residents of Brighton, Eastbourne and Hastings regardless of their health.
The warning comes as Britain is expected to enjoy the hottest day of the year so far, with some areas of Eastern England predicted to reach highs of 22c (71.6F).
The Department for Environment, Food and Rural affairs (Defra) said it predicts the pollution to have cleared by the weekend (11 April).
Dr Sotiris Vardoulakis, head of the air pollution and climate change group at PHE's Centre for Radiation, Chemical and Environmental Hazards said: "While most people will not be affected by short term peaks in air pollution, some individuals, particularly those with existing heart or lung conditions, may experience increased symptoms.
"On occasions where levels are high, adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.
"Some parts of the country may record 'very high' levels of air pollution. PHE is advising people in those areas to reduce physical exertion, particularly outdoors, especially if they experience symptoms such as a cough or sore throat.
Dr Samantha Walker, director of research and policy at Asthma UK, added: "Two thirds of people with asthma find that air pollution makes their asthma worse, putting them at an increased risk of a potentially fatal asthma attack.
"When air pollution is high it's vital people with respiratory conditions, including asthma, check air pollution forecasts, carry their reliever inhaler with them at all times, and ensure that they are taking their preventer inhaler every day because this will help build resilience to asthma triggers like air pollution."
In March, parts of the UK were hit by a toxic smog which engulfed the country after it drifted over from France. Campaigners believe today's pollution will be the most serious since the Saharan Dust cloud which the UK in 2014.
Simon Birkett, director of campaign group Clean Air in London, said: "If there is one lesson this week, it's that we must follow the lead of cities like Paris that are issuing public health warnings, restricting traffic and putting forward ambitious plans to triple cycling rates within five years."