A "very high" air pollution warning has been issued for London for the first time under a new alert system.
Warnings are being issued at bus stops, roadside signs and Tube stations under the new system set up by London Mayor Sadiq Khan.
The rise has been attributed to cold, calm and settled weather, meaning winds are not dispersing local pollutants.
The mayor said "the shameful state of London's toxic air" meant he had to trigger the alert.
"This is the highest level of alert and everyone - from the most vulnerable to the physically fit - may need to take precautions to protect themselves from the filthy air," he said.
A spike in pollution on Sunday was the highest level recorded since April 2011.
Current concentrations of PM10 - tiny airborne particles caused by the burning of fossil fuels - are more than double the legal limit.
The air quality index numbers run from one (lowest) to 10 (highest). The current levels have been rated as 10.
Dr Gary Fuller from the Environmental Research Group at King's College London said episodes of pollution at this level "happen a few times a year".
Susan Boix, from Erith, South London
While walking the dogs today I felt so sick from the pollution and now have a sore throat.
It was so bad I felt like I was in an enclosed space with a car engine running.
I had to pull my sweatshirt up to cover my mouth and nose. I've never experienced it like that before.
Now and again it's bad but not like today. You could taste the pollution.
The last time pollution reached this level was early last month, according to pollution monitoring stations run by King's College London.
However, a spike in pollution levels on Sunday "when there wasn't much traffic on the road, was significant" Dr Fuller said.
In recent weeks several "high" alerts have been issued.
The current weather conditions, coupled with an "unusually high amount of domestic wood burning", has led to the highest pollution alert being issued.
The government's committee on the medical effects of air pollutants (COMEAP) advises adults and children with lung problems, adults with heart problems and older people, to avoid strenuous physical activity.
People with asthma may find they need to use their inhaler more often.
Pollution levels are expected to continue into Tuesday.
Have you made changes following the air pollution alert? Are you staying indoors or working from home? Have you made other changes? You can email firstname.lastname@example.org with your experiences.
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