Oxford recorded its highest possible air-pollution rating after fireworks and bonfires took place nationwide during the weekend.
The city received a purple colour-coded Very High rating of 10 out of 10 on the Daily Air Quality Index.
This represents the maximum band in the Government’s air-quality scale.
Elevated pollution has affected parts of the south east, where weak winds and persistent fog have restricted the dispersal of contaminated air.
The Met Office, which supplies forecasts incorporated into UK AIR, reported that areas with moderate or high pollution had developed, particularly beside busy roads, around urban centres and where fog remained slow to clear.
Very High readings were also detected in some urban locations, while locations away from fog and large cities generally experienced low pollution.
Health guidance recommends reducing demanding outdoor activity in places where pollution is high, especially if coughing or a sore throat develops.
The risk is greater for children and adults with lung conditions, as well as adults with heart problems.
People with asthma may require more frequent use of their reliever inhaler when pollution reaches these levels.
Government measurements use a scale running from one to 10.
A rating of 10 falls within the Very High category, while figures from seven to nine are classified as High.
The assessment considers five pollutants: PM2.5 particles, PM10 particles, ozone, sulphur dioxide and nitrogen dioxide.
The Government’s air-quality service for Oxford provides access to the latest conditions.