A small, portable humidifier costing as little as £50 could significantly reduce the risk of catching Covid, flu and other viruses in the cold winter months by safely disinfecting them, according to a leading chemist.
Research by Professor Richard Zare, of Stanford University, shows that that moisture-producing humidifiers could usefully be incorporated into all sorts of areas where people can pass on a virus, from care homes and hospitals to schools, offices and even the home.
But this would only work in the winter months, when indoor heating dries out the air. In the summer months there is already enough humidity.
Professor Zare also advises farmers to put humidifiers around chickens, turkeys and other farm animals to reduce the risk of spreading bird flu and other viruses which, in turn, would decrease the danger of a new strain developing that could pass among humans and create another pandemic.
He has discovered that hydrogen peroxide – a caustic substance used for disinfecting surfaces and bleaching hair – spontaneously forms in microscopic droplets of ordinary water in tiny but effective amounts that are harmless to humans.
And he has found that when it is a bit more humid the effect is greatly enhanced because it slows down the rate at which the water droplets evaporate, giving them longer to kill the virus.
This suggests that humidifiers could significantly reduce the volume of harmful particles in the room, he told the American Association for the Advancement of Science conference in Washington.
Some people already have humidifiers in their homes for a variety of reasons, including help with coughs, reduce snoring, keep skin and hair hydrtated, help houseplants and prevent static electricity building up.
“The public health implications of this work are clear – controlling indoor humidity is a new way to stop the spread of viral respiratory infections,” said Professor Zare.
“If links are further borne out, simply adding humidifiers to heating, ventilation, and cooling systems could lessen disease transmission.”
Professor Zare said his findings help explain why colds and flu, in particular, are more common in the winter.
In the summer, indoor humidity is higher because the warmer outside air is more humid (containing more water) while the heating is unlikely to be on, meaning there will be larger amounts of hydrogen peroxide with enough time to kill viruses. By contrast, in winter, when the air inside buildings is heated and its humidity lowered, the droplets evaporate before the reactive oxygen species can act as a disinfectant, he argues.
“Viral respiratory infections are seasonal, peaking in the winter time when the indoor air is heated and dried out (low relative humidity) but falling in the summer time when the indoor air has a relative humidity much like the outside air, often ranging between 40 per cent and 60 per cent,” he said.
He points to a famous comment made by Scott Dowell, a leading epidemiologist at the Bill and Melinda Gates Foundation, in 2020: “If we knew what suppressed influenza to summertime levels, that would be a lot more effective than any of the flu vaccines we have'”, arguing that his work has gone some way to answering that question.
It’s not clear how much a humidifier would reduce the risk of catching a virus but Professor Zare expects it to be substantial – not least because of the fact that infections are so much lower in the summer.
“I do not know for sure, but in summertime when the relative humidity is high the incidence of viral respiratory infections falls off markedly,” Professor Zare says.
Meanwhile, a study in the journal PLOS ONE, which humidified some classrooms and not others, found that “there were 2.3 times as many flu cases in the control [non-humidified] rooms compared to the humidified rooms”.
“This suggests the future potential of artificial humidification as a possible strategy to control influenza outbreaks in temperate climates,” the study concluded.
Another study, by the Massachusetts Institute of Technology in November and published in the Journal of the Royal Society Interface, found that, the humidity “sweet spot” of 40 to 60 per cent (see explainer) “is associated with a better outcome [for Covid], meaning fewer deaths and a deceleration of the pandemic.”
Meanwhile, research led by Kindai University in Japan and published in October in the journal Indoor Air, found “strong evidence that the decreased relative and absolute humidity in the indoor environment are positively and significantly associated with increased risk of upper respiratory symptoms related to the building.”
Simon Williams, of Swansea University, who has been studying the way Covid has affected people’s behaviour, said: “If research suggests that this could lead to reduction in Covid and other virus transmission, then further research, investment and evaluation would be important.
“I think clean indoor air is one of the most important things we should be investing in to help curb not just covid, but also other seasonal respiratory viruses.”
“We heard a lot about the ‘Swiss cheese’ approach to public health during the pandemic – which is where the more layers of protection we have the more we reduce individual and population risk against Covid and other respiratory infections. Ideally, the more systemic protections, things like vaccinations and ventilation, the less we need to rely on what we call high-cost measures like social distancing and self-isolation (these are high cost because they involve sacrifices).
“And so investing in things like HEPA filters, air purifiers and humidifiers can contribute to a multi-pronged approach to reducing transmission.”
How to go about humidifying your property
With high-profile stories about damp and black mould in the news people could be forgiven for thinking you need less, rather than more, moisture in the home.
But Professor Richard Zare, of Stanford University, argues that, in the cold winter months, when the heating is on, increasing the amount of water vapour in the air can very often be done without increasing the risk of damp and mould because the warmth from the heating dries the air out.
He recommends a “relative humidity” rate of 40 to 60 per cent as being the safe, “sweet spot” and recommends measuring the humidity in the room with a cheap hygrometer, costing less than £10, to be sure that a humidifier is needed and that it is working at the correct levels.
Relative humidity is the term for how much water vapour is actually in the air compared to how much it can hold. Scientists often compare it to a sponge: At 100 per cent, the sponge is totally soaked; at 50 per cent, it holds half as much water.
“Humidity levels affect the evaporation rate of microdroplets, or aerosols, in the air, and relative humidity values of 40 to 60 percent provide enough time to kill the viruses. This killing is promoted by the presence of reactive oxygen species [hydrogen peroxide] in water-containing microdroplets,” he said.
Most people are comfortable between 30 and 50 per cent relative humidity, and an airplane cabin is at around 20 percent relative humidity, experts say.
Meanwhile, opening windows or other types of ventilation remains important, where possible, even with humidifiers as they can complement another, experts say.
Adriaan Davidse, an independent researcher who has worked with Professor Zare on the effects of humidity on viruses, explains how he would set things up.
“What you have to do is get a cheap hydrometer or a few that you can put in the rooms in your house you spend most time in. Then just become aware first of how much the relatively humidity (RH) varies and how opening windows and heating affect the RH. If it consistently drops below 40 per cent then you should use a humidifier to try to bring it up to 40 per cent again. And in summer you may need a dehumidifier (basically similar to an AC) to keep it below 60 per cent. There is overwhelming evidence that keeping RH between 40 and 60 per cent significantly decrease viral respiratory infections.”
“I am somewhat obsessed about maintaining RH at 50 per cent, not just because it prevents the infection to spread if one of us gets sick, but because our overall health is significantly better. We sleep better, and we don’t have dry skin in winter at all, and a host of other benefits.”