Smog over parts of England, Wales and Northern Ireland has prompted a warning from Public Health England.
The Department for Environment said there was moderate to high levels of the pollution, which is caused by traffic fumes and other sources.
Air pollution has also blown in from Europe and Public Health England said the conditions could affect people with lung or heart problems or asthma.
Defra said levels were due to fall to moderate or low levels by Friday.
Smog is formed when pollutants, dust, pollen and other particles that would normally be dispersed by winds are prevented from escaping from the layer of cool air that is trapped at the Earth's surface.
Sotiris Vardoulakis, head of Public Health England's air pollution and climate change group, said: "While most people will not be affected by short term peaks in air pollution - some individuals, particularly vulnerable groups such as those with existing heart or lung conditions, may experience increased symptoms."
According to Defra, south Wales and South West England saw the highest levels of pollution in the UK on Thursday, reaching eight out of 10 on its pollution index.
The West Midlands, Yorkshire and Humber, North East England and Northern Ireland also recorded high levels - graded as seven out of ten on the index.
North-west England, the East Midlands and parts of London had seen moderate levels - six out of 10 - while Scotland had seen low levels of pollution, Defra said.
Deborah Waddell from Asthma UK warned that people with breathing problems needed to be careful.
"We could expect that people today would have more asthma symptoms and every person that has an asthma attack today could have a potentially have a fatal asthma attack so we need them to be vigilant," she said.
Steve Cleaton from the BBC Weather Centre said: "The good news is that air quality is forecast to gradually improve through the course of the remainder of Thursday and throughout Friday.
"Weak weather fronts will cross the country from the northwest introducing clearer Atlantic air."
High levels of air pollution were reported in Paris, France, on Wednesday.