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 will trial using air purifiers and UV lights in a bid to make classrooms even safer in fight against Covid
Schools in England have installed air purifiers and UV lights in classrooms as part of a Covid-fighting trial.
The equipment has been installed in 30 schools in Bradford to assess whether they curb the spread of the coronavirus and other respiratory diseases.
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, which will consider the final results from the study once available.
It is being conducted by the Centre for Applied Education Research and could lead to the technologies being rolled out in schools from 2022.
The randomised trial will consist of a third of schools equipped with high-efficiency particulate air (HEPA) filters, a third with UV purifiers and a third without any equipment to serve as a control group.
Volunteers were either asked to repeat the alphabet three times while taking part in the study, or sing 'happy birthday' three times.
They found the amount of virus Alpha-infected people exhaled was 43 to 100 times higher compared to earlier strains, depending on which type of particles the experts looked at.
When researchers adjusted for this, they claimed levels of Alpha circulating in the air were still 18 times higher than earlier versions of Covid.
But when wearing a mask, infected people exhaled 48 per cent less of the virus, they found.
The finding was true for both for people wearing surgical or cloth masks.
The researchers are yet to enroll someone who is infected with the Delta strain.
But they claimed their findings suggest there will be an 'additional large increase in aerosol shedding' in people who have that strain.
Despite air transmission of the virus being widely recognised as a main way the virus spreads, studies on how well it spreads when exhaled and the effect of face masks were previously lacking, the researchers said.
Researchers said their study shows that a 'layered approach' that includes improved ventilation, UV air sanitation and tight-fitting masks – along with the vaccine rollout – is 'critical to protect people in public-facing jobs and indoor spaces', the researchers said.
The Government announced last month that it installed air purifiers and UV lights in 30 classrooms as part of a trial to determine whether they curbed the spread of the virus, with results expected later this year.
Dr Milton said: 'Our study provides further evidence of the importance of airborne transmission.
'We know that the Delta variant circulating now is even more contagious than the Alpha variant.
'Our research indicates that the variants just keep getting better at travelling through the air, so we must provide better ventilation and wear tight-fitting masks, in addition to vaccination, to help stop spread of the virus.'
Dr Jennifer German, an assistant clinical professor in applied environmental health at the university and co-author of the study said: 'The take-home messages from this paper are that the coronavirus can be in your exhaled breath, is getting better at being in your exhaled breath, and using a mask reduces the chance of you breathing it on others.'