POLLUTION from a smog cloud caused by sand from the Sahara Desert has hit Bolton.
Bolton's air quality index has reached 5, a "moderate" level according to the Met Office, considerably higher than the town's normal "low" level of 2.
Public health chiefs are warning people have been warned to cut down on outdoor exercise as the dust cloud heads northwards today.
The warning has been directed at anyone suffering the effects of high pollution levels - including sore eyes, coughs and sore throats.
The worst pollution has affected London and southern England today, with "high" and "very high" levels recorded.
Increased air pollution is expected to afflict Bolton again tomorrow, with forecasters again predicting an index of 5, while "high" pollution is forecast as nearby as Merseyside and the Wirral.
Experts say the sand clouds can be carried thousands of miles in dense form, and public health bosses have said England is currently experiencing its worst ever levels of air pollution.
Manchester's air quality has been forecast by the Met Office as 4 today 5 tomorrow.
Asthmatics have been told they might need to use their blue reliever inhalers more often as they could be prone to attacks today and over the next few days.
Other people with lung and heart problems, and those who are older, should also avoid strenuous exercise or activity, experts say.
High levels will move north over much of coastal north west England, to south west Scotland and the north east of Northern Ireland.
Defra ranks air pollution from 1 to 10, with 1 being the lowest and 10 the highest.
Across most of England, moderate to high air pollution levels are forecast, with level 10 expected in parts of East Anglia and the East Midlands tomorrow.
Paul Cosford, director of health protection at PHE, told BBC Radio 4's Today programme: "Clearly this is a serious issue but it's important that we don't get it out of proportion.
"It's a small number of days of very high air pollution levels. The pollution will go down towards the end of the week.
"For the vast majority of people, they will suffer no harm as a result. We may notice sore eyes, coughs, sore throats and perhaps a little bit of a wheeze if we are taking physical activity outdoors.
"If that occurs, it is sensible just to reduce during these high pollution episodes the amount of physical activity."
"The most important advice is for those people who are vulnerable - people who have lung disease or heart disease and particularly asthma.
"That's where the advice - if you are in a very high-polluted area, to reduce the amount of strenuous physical activity that you are taking outdoors - is sensible just for these few days."
The elevated pollution levels have been caused by a combination of light south-easterly winds, the continental air flow and dust which has blown up from the Sahara desert.