A Sage scientist has urged people to ventilate their homes this winter to help cut the risk of Covid-19.
Professor Shaun Fitzgerald, who sits on the Sage environmental working group, said opening a window at home was almost as vital as social distancing and washing your hands.
The University of Cambridge scientist told the Times that it is especially important if people are visiting.
But he warns that you have to be careful - as if it gets too cold inside, that carries its own health risks.
“The bigger the temperature difference between the inside and the outside, the more airflow you will get,” Professor Fitzgerald said.
“So you can actually get away with not quite as much opening area in winter.”
But he added: “There’s a benefit to a bit of extra air but it’s diminishing returns and if you freeze people, you start running up against other health issues."
He also said air purifiers could help - but said ventilating with fresh air was better where possible.
“It’s much better to ventilate a space using fresh air if you can,” Professor Fitzgerald said.
“But if you can’t, then these are worth looking at. I wouldn’t turn there first but I’m not saying don’t turn there at all.”
His words on window-opening are supported by the Health and Safety Executive.
Their website states: "Employers must, by law, ensure an adequate supply of fresh air in the workplace and this has not changed.
"Good ventilation can help reduce the risk of spreading coronavirus, so focus on improving general ventilation, preferably through fresh air or mechanical systems.
"Where possible, consider ways to maintain and increase the supply of fresh air, for example, by opening windows and doors (unless fire doors).
"Also consider if you can improve the circulation of outside air and prevent pockets of stagnant air in occupied spaces. You can do this by using ceiling fans or desk fans for example, provided good ventilation is maintained.
"The risk of transmission through the use of ceiling and desk fans is extremely low providing there is good ventilation in the area it is being used, preferably provided by fresh air.
According to the Sun, studies on other infectious diseases such as Sars and influenza have discovered a reduction in the spread of the virus in buildings that have better ventilation and air movement.
The Centres for Disease Control and Prevention, the prominent public health body in America, also says that good ventilation indoors can help reduce the risk of the virus spreading.
It says that houses, workplaces, schools and restaurants should "increase ventilation by opening windows or adjusting air conditioning" in a bid to combat Covid.
The World Health Organisation offers similar advice.
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It says that poorly ventilated buildings can increase the risk of disease transmission.
The UK government's 'Hands. Face. Space' campaign urges the public to continue to wash their hands, cover their face and make space to control infection rates and avoid a second peak.
Speaking about the government's campaign, the Chief Medical Officer Professor Chris Whitty said: "As we approach winter and inevitably spend more time indoors, we need the public to keep following this important advice to control the spread of the virus.
"‘Hands. Face. Space’ emphasises important elements of the guidance we want everybody to remember: wash your hands regularly, use a face covering when social distancing is not possible and try to keep your distance from those not in your household.
"Following these simple steps could make a significant difference in reducing the transmission of COVID-19 and help protect you and your friends, colleagues and family from the virus."
Speaking when the virus outbreak first swept the globe, Joseph Minott, executive director of the Clean Air Council, a Philadelphia-based environmental nonprofit, said opening a window was really important to get pollutants out of the house.
“The thing that I always suggest in a non-professional way is to open the window and get some fresh air,” he said. “It’s something my mother swore by and it’s something that I do also — to sort of get the stale indoor air out and the fresh air in.”