Air pollution levels have reached "very high" or "high" in eight regions across the UK, the government has said.
People with lung problems, adults with heart problems, or older people should avoid strenuous physical activity in the most badly hit areas, it said.
In London, the Mayor issued a separate "very high" alert about "filthy" air.
Northern Ireland, south-west England, West Midlands and Yorkshire and Humberside all had "high" pollution levels in some areas, according to Defra
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