Pollution from neighbouring EU countries is set to last for the next three days with the hit the southern half of the UK hit yesterday.
Today, Wales will be worst affected with the smog drifting into the north as well.
The Government already has a moderate pollution warning, although it is expected to peak at high on Sunday.
It has urged anyone who suffers breathlessness, asthma, and other lung problems, or those with a weak heart, to avoid breathing in the smog and resist taking part in strenuous activity.
In some cases they are being urged to stay indoors.
A five-day forecast shows low pollution levels today, with much of the UK turning to moderate as the clouds sweep over from France.
It will drift north today and tomorrow, but a second wave coming in from the continent on Sunday will reach as far north as the Scottish highlands.
Areas of Devon and Cornwall on the coast will also be under a high pollution alert.
The pollution levels will be compounded by high pollen count warnings from the Met Office over the same days, meaning hayfever sufferers are likely to experience worse symptoms.
A Defra spokesperson said when pollution levels are high “adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms”.
It added: ”People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.”
Even those without existing conditions should be aware to symptoms.
They added: “Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.”
Graham Curtis, an expert on air purifiers at Vax, said: “Dirty air from the continent hit the UK on Thursday.
“This is a problem that’s not going to go away anytime soon. It’s the latest in an increasingly common series of health warnings related to poor air quality.
“As air quality drops the impact on health will increase.
“Taking steps to reduce outdoor activity during high air pollution times and cleaning your indoor air with an air purifier are all positive steps to help reduce the effects for you and your family.”
During the London mayoral campaign Tory candidate Zac Goldsmith and Labour rival Sadiq Khan have been urged to strengthen plans to tackle toxic air which was recently blamed for around 9,400 lives lost a year in the capital.
Campaigners warn the Government’s air quality masterplan could fail to dramatically reduce pollution and more than 350 areas in London could breach EU nitrogen dioxide limits by 2020.
Simon Birkett, founder of Clean Air in London which compiled the figures, said: “The only subsequent measure proposed by Boris Johnson to address this catastrophe is his so-called Ultra Low Emission Zone which is too small, too weak and too late in September 2020.
“Worse, it will cover only a tiny fraction of these breaches and worsen many of them.”
A City Hall spokesperson said it was carrying out some of the “most ambitious and comprehensive measures in the world” to improve London air quality.
A DEFRA spokesman said: “Our plans clearly set out how we will improve the UK’s air quality through a new programme of clean air zones, which alongside national action and continued investment in clean technologies will create cleaner, healthier air for all.”