Air quality monitoring stations across the UK measure particles of dust in the air that can be breathed into the lungs.
ITV News Correspondent Rupert Evelyn visited a monitoring station in King's Lynn in Norfolk:
Health manager Dave Robson of King's Lynn and West Norfolk Council told ITV News the levels at the local station were "above what we'd normally see".
Higher numbers of complaints about breathing difficulties are thought to have been brought on by record pollution levels in recent days.
Air pollution in the east of England reached the top of the scale as a 'perfect storm' of conditions blanketed parts of the country.
Our region is one of the worst-affected by "very high" levels of air pollution today, with much of the East stuck under a cloud of smog.