Defra's latest air pollution levels for Greater London are 10/10 'very high'.
The levels are a result of a combination of high pressure trapping in stale air and pollution blowing over from the continent.
The pollution can be seen as the haze in the air.
Certain types of air pollution are proven to have a damaging effect on health. It's estimated pollution contributes to 4,000 early deaths a year in London.
This weekend Paris transport authorities are making public transport free over the weekend due to health risks from the air pollution.
The Green party says London's Mayor should take a leaf out of Paris' book and take action, starting with official smog warnings.
– Jenny Jones, Green Party
“The Mayor cannot sit idly by this weekend while millions of Londoners are exposed to pollution that can permanently damage our health.
He must alert the 2,500 schools in London that are close to polluted roads and consider closing the worst roads to all but buses and essential traffic. Like Paris, he needs to think big."
Is fog related to pollution?
Fog can (and does) form in clean air but the presence of pollutant particles in the air can make fog more likely.
Radiation fog, which is caused by clear skies and calm conditions, is more common when high pressure is in charge.
High pressure (which we have at the moment) also contributes to higher levels of air pollution as stale air gets trapped.
How long will the high levels last?
Things should gradually improve over the weekend. We have a change of wind direction bringing in cleaner North Atlantic air rather than dirty air from the continent.
The breeze will also pick up stirring up the stale polluted air and mixing it with cleaner air in the atmosphere.
Next week low pressure takes charge once more.
That's good news for air pollution levels but bad news for sun lovers.
We're likely to see more cloud, more light rain and breezier conditions.
The current pollution episode comes whilst a public consultation is underway to canvass Londoners on their views about air pollution in London. You can see more here.