The number of people calling 999 with breathing difficulties has spiked in parts of the UK as pollution levels reached record highs.
In London - which has seen 'very high' levels since Wednesday - emergency services saw a 14% rise in such complaints yesterday and higher volumes today.
Dr Fenella Wrigley from the London Ambulance Service has witnessed a "definite increase" in such issues:
It wasn't simply felt in the capital, however - ambulance services in the West Midlands, East of England and north-west also reported a surge in calls thought to be related to the smog.
On asthma sufferer said:
Atmospheric pollution is determined by the concentration of pollutants in the area, such as Ozone, Sulphur Dioxide, Nitrogen Dioxide and tiny specks of matter called particulates.
Especially hazardous are the smaller particulates called PM2.5, which are 100 times thinner than a human hair.
Gary Fuller, air pollution scientist at Kings College London told ITV News the dust particles in the air can brought right down into people's lungs.
Pollution levels remained high across England and South Wales in the evening on Thursday, as this graphic from the Department for Environment, Food and Rural Affairs shows.
However, it is expected to clear overnight as cleaner, fresher south-westerly winds blow in - which should ensure a respite for those who have been suffering.