The coronavirus is evolving to become better at spreading through the air, scientists say.
People infected with the Alpha variant, which triggered Britain's devastating second wave in January, expel up to 100 times more of the virus into the air than those who caught the original Wuhan strain.
That is according to researchers from the University of Maryland, who say Delta likely transmits even easier through the air.
Lead author Dr Don Milton said the findings indicate variants 'just keep getting better at travelling through the air'.
The study, which was based on monitoring the air that infected volunteers exhaled into a machine, also claimed cloth and surgical masks can halve the amount of virus that an infected person exhales.
Participants in the study breathed into a machine called a Gesundheit-II - for 30 minutes without wearing a mask and 30 minutes while wearing a mask to determine how face coverings impact the spread of the virus
The experts focused on the Alpha strain – which emerged in Kent last autumn and became dominant in the UK by January and in the US by March.
This is because it was the prevalent strain when they carried out their research.
Delta, which was first spotted in India last October, quickly overtook Alpha to become the prevailing strain in Britain and America by July.
The study, published in the journal Clinical Infectious Diseases, examined 49 people infected with Alpha and earlier strains of the virus – who were asymptomatic or had mild symptoms – between May 2020 and April 2021.
Participants breathed into a machine called a Gesundheit-II for 30 minutes without wearing a mask and 30 minutes while wearing a mask.
Schools in England have installed air purifiers and UV lights in classrooms as part of a Covid-fighting trial.
It will also evaluate how feasible it is to implement the technologies in primary schools and is expected to yield its first results by the end of the year.
Schools Minister Nick Gibb said in August whether the scheme is rolled out to other schools nationwide depends on the result of the trial.
The study is backed with £1.8million of funding from the Department of Health and Social Care,