An unhealthy mix of dust particles from the Sahara Desert along with pollution from the big cities in Western Europe has cut visibilities across the UK this week. In addition, light surface winds have let the toxic soup of ingredients in place, rather than sweeping them out to sea.

The worst of the pollution has been across southeast England, including London, where air quality has deteriorated to level 10 on the 10-point scale - the worst-possible reading. Levels are determined by the amount of five pollutants in the air — ozone, sulphur dioxide, nitrogen oxide and two types of particulate matter. Such hazardous air quality levels can be harmful to those with compromised immune systems, respiratory ailments including asthma, and the young and elderly, and the UK typically only experiences levels this high about five times a year.

According to BBC News, the London Ambulance Service reported a 14% increase on Wednesday in emergency calls related to breathing difficulties. The BBC quoted British Prime Minister David Cameron as saying the atmosphere has been "unpleasant."

"I didn't go for my morning run this morning; I chose to do some work instead, but you can feel it," Cameron said.

As Mashable reported Wednesday, a large dust storm in the Sahara Desert vaulted dust to high altitudes, where it was picked up by strong winds and carried all the way to the UK. Typically, dust from the Sahara settles in southern Europe, including Spain, Portugal and Greece. The dust, plus pollution from factories and vehicles, has elevated pollution levels and blanketed London and other parts of the UK in a milky haze.

Conditions are expected to improve by the weekend, as low level winds increase and scour out the pollution, the UK Met Office said.