Mild winds from Europe have brought high levels of airborne contamination sparking warnings for people with breathing problems to take care.
France’s air pollution monitoring system Prev’Air has reported a build up of smog around the UK which is expected to last through the week.
Although it has not yet been flagged on Defra’s pollution forecast readings show high levels of PM10 - soot, metal and chemical - particles.
Western regions are currently seeing levels of around 80 microns per cubed-metre of air on a scale of zero to 125.
Levels are expected to drop off over the coming days though still hovering around 50 - high enough to worry health experts.
Graphs show levels of pollution over the UK
Asthma UK warned high pollution levels common at this time of year could trigger deadly asthma attacks.
A spokesman said: “There are reports from Prev’Air, France’s air pollution monitoring system, that there is a large build-up of smog over and around the UK.
“This is yet to be reported by DEFRA’s pollution forecasting but they are recording moderate to high pollution In urban areas.
“Historically the middle of March is when we start to see pollution spikes.”
Chief Executive Kay Boycott added: “Two thirds of people with asthma find that air pollution leaves them fighting for breath, putting them at increased risk of a potentially life-threatening asthma attack.
“When air pollution levels are higher it’s vital that people with asthma check pollution forecasts; carry their reliever inhaler with them at all times; and have an up to date written asthma action plan so they know what to do if their symptoms get worse.”
It comes as weather forecasters say Britain is likely to enjoy spring-like weather for the next 10 days at least.
Temperatures are expected to nudge the 20C mark by Easter with plenty of blue skies and sunshine around over the next fortnight.