Air pollution in the east of England reached the top of the scale as a 'perfect storm' of conditions blanketed parts of the country.
Towns like Norwich were shrouded in a smog that experts have warned could threaten the health of thousands of people, particularly those with lung and heart conditions.
Stan Mumby from Lincoln said the conditions had him fighting for every breath, leaving him "absolutely shattered".
Asthmatics have been warned of the need to use their blue reliever inhalers more often as they could be prone to attacks over the next few days.
Around two-thirds of the 3.6 million people with asthma find that air pollution makes their asthma worse.
Dr Jill Meara from Public Health England says shortness of breath, a cough or sore eyes might be among the signs that people should avoid going outside while pollution levels remain high.
The smog is a cocktail with three ingredients. The first: dust particles swept through the upper atmosphere from storms in the Sahara
Those particles then sank into the lower atmosphere when they reached Britain, mixing with the other two ingredients: pollution from cars and factories, and light winds that trapped it on our shores.
The impact of the pollution has not just been felt, but seen too - with thick clouds covering the skies in parts of Cambridgeshire and Essex.
Cars, too, could be seen blanketed in dust brought over from the African desert.
Tomorrow, high levels of pollution are forecast for East Anglia, the Midlands, Lincolnshire, eastern parts of Wales, through the Wirral and the north coast of Wales.
High levels will move north over much of coastal north-west England, to south-west Scotland and the north-east of Northern Ireland.