The science behind the summer pollen count
5 July 2015 Last updated at 08:12 BST
This week saw the highest temperature for a July day ever recorded in the UK, but while many enjoyed being outdoors, others were suffering.
It's peak season for pollen at the moment and with an estimated 31 million people suffering from hayfever, the pollen count is an important part of the weather forecast.
But how do they work it out?
Jenny Lawrence reports.