Air pollution has reached the highest possible level in some parts of the country, with "very high" readings reported in London and Kent.
High levels of smog are also forecast for East Anglia, the Midlands, Lincolnshire, the Wirral and the north coast of Wales and a health warning remains in force for millions of vulnerable people.
The elderly, people with asthma and those with lung and heart conditions are being told to take precautions, with forecasters warning the smog will not clear until Friday.
Sky News Correspondent Lisa Dowd, at the Acute Medical Unit in Solihull, near Birmingham, said the clinic had seen a "slight increase" in the number of people coming in with respiratory problems possibly linked to the poor air quality.
Karen Lytton, who was among the patients admitted, said: "The breathing problems I suffer with have been getting worse.
"It's been getting harder to get about. Normally I'd be able to walk to the shops but yesterday I had to catch a taxi instead."
Parts of England are experiencing the highest level of air pollution ever recorded by the Department for Environment, Food and Rural Affairs (Defra), in part due to dust from the Sahara.
Level 10 readings were reported at monitoring sites in Harrow, northwest London, and Rochester, Kent.
Tests by an independent monitoring company in Tewkesbury, Gloucestershire, revealed there were 130 micrograms of particulates - very fine particles, primarily from diesel-powered car and industry - in every cubic metre of air.
That figure is more than six times higher than normal and two and half times the European limit of 50 micrograms.
The plummeting air quality - caused by a cocktail of dust from the Sahara desert, domestic pollution and European emissions, coupled with low southeasterly winds - has led charities and health bodies to warn of possible health risks.
