A cloud of toxic smog over Britain combined with air pollution blowing in from Europe has caused a health alert, and could be an obstacle to those hoping to get a glimpse of tomorrow's rare solar eclipse.
Still air and high pressure mean the pollution is building up, getting trapped in the atmosphere and causing problems.
The current cloud is being caused by a combination of home-grown smog, mixed with air pollution from the continent, The Department for Environment, Food and Rural Affairs (Defra), said.
Although it is expected to have mostly dissipated by tomorrow, it is thought that a haze could still remain, reducing visibility of the eclipse.
Experts have warned that the polluted air could cause fatal asthma attacks and have advised the elderly and those with health problems to be cautious.
Increased levels of toxic gases, such as nitrogen dioxide and sulphur dioxide, mean there is a lower concentration of oxygen in the air breathed, said Professor Liz Bentley, Chief Executive of the Royal Meteorological Society.
She added: "There are a mixture of gases that make up the current pollution that we have got.
"There are pictures coming in from Paris that show it is very hazy, and as well the clouds which will block the sun for tomorrow's eclipse, the haze from the pollution is not going to help."
Prof Bentley explained that some of the air pollution is being caused by industry in the UK, and that some of it is coming from the continent, as far as Russia.
Kay Boycott, chief executive of Asthma UK, said: "Two thirds of people with asthma find that air pollution makes their asthma worse, putting them at an increased risk of a potentially fatal asthma attack.
"When air pollution is high it's vital people with respiratory conditions including asthma check air pollution forecasts, carry their reliever inhaler with them at all times, and ensure that they are taking their preventer inhaler every day because this will help build resilience to asthma triggers like air pollution.
"People with asthma have told us that on days when air pollution levels are high they feel that they can't even leave the house for fear that it will trigger an attack."
According to Defra large parts of England will see air pollution climb to moderate levels today while parts of the West Midlands, Yorkshire and Humberside, the South West and Wales could climb to high or even reach eight out of 10.
A Defra spokeswoman said: "Winds bringing in pollution from the continent, combined with locally generated pollution and still weather conditions has led to some high pollution measurements across the UK. Levels are expected to return to low by Friday."
She added that high levels are not seen very often, and that it is not unusual to see an increased level of pollution in the spring.
Met Office spokeswoman Nicky Maxey said high pressure and still air are causing the pollution patch to hang around.
However, she added that it was an "improving picture" with winds starting to blow in a north-easterly direction.
Explaining the chances of people catching a glimpse of the eclipse, she continued: "It is fairly cloudy tomorrow, but there is a possibility that there might be some breaks in the cloud, but it is not really possible to say where. However, most people should still be able to experience it becoming dark."
Defra said that cloud cover is more likely to block the view of the solar eclipse than the air pollution, which is expected to return to low levels tomorrow.
The British Lung Foundation (BLF) has advised that there is no need for people with lung conditions to panic as there are measures they can take to minimise the chance of worsening symptoms.
Dr Penny Woods, chief executive of the charity, said: "For instance, avoiding commuting near main roads or other pollution hotspots during rush hour is preferable if possible - even if you're in a car, as pollution can still seep in even with the windows wound up. Similarly, exercising in an air-conditioned indoor space is better than exercise near polluted spots or no exercise at all."
She added that anyone feeling more breathless or experiencing more coughing or wheezing during periods of high pollution should contact their GP or the BLF, rather than assuming it will just go away.
"Anyone experiencing an asthma or COPD (chronic obstructive pulmonary disease) attack, or otherwise really struggling to breathe, should immediately dial 999," Dr Woods said.