As the wind blew hard and the temperature dropped, my grandmother urged me to come inside. “You’ll catch a cold there,” she said.
Of course, freezing to death is possible at low temperatures. But doctors and other health experts have long emphasized what cold does not cause colds. That said, winter is definitely cold and flu season. This is also the period when COVID-19 spreads more.
But if the common cold doesn’t matter, why does the spread of so many respiratory viruses peak during the season?
“I’ve been researching this question for the last 13 years,” says Lynsey Marr, a civil and environmental engineer at Virginia Tech in Blacksburg who studies airborne viruses. “The deeper we go, the more I realize that we don’t know anything [і] the more there is to find out.”
We are not alone with her. “This winter seasonality has puzzled people for a very long time; frankly, thousands of years,” says Jeffrey Shaman, an infectious disease researcher who directs the Climate and Health Program at Columbia University’s Mailman School of Public Health.
He said there is some evidence that shorter days in winter may make people more susceptible to infection. Less sunlight means people produce less vitamin D, which is needed for some immune responses. But this is only one part of the puzzle.
Scientists are also investigating what other factors may play a role in making winter an unpleasant season.
The disease can spread internally.
Instead, my grandmother’s well-intentioned urging to come in from the cold may have increased my risk of getting sick.
Colds, flu, and respiratory syncytial virus, or RSV, are illnesses that are more common at certain times of the year when people spend more time indoors. This includes winter in temperate climates, where there are distinct seasons, and rainy seasons in tropical areas. COVID-19 as well more spread indoors than on the street.
These diseases are caused by viruses that are transmitted mainly through breathing in the form of small droplets known as aerosols . It’s a mindset shift. Until recently, many scientists believed that such viruses spread mainly through touching contaminated surfaces.
“When you’re outside, you’re in a well-ventilated area,” says David Fisman, an epidemiologist at Toronto’s Dalla Lana School of Public Health. Viruses that are exhaled are quickly diluted by clean air.
But aerosols and the viruses they contain can accumulate inside. “When you’re in a poorly ventilated space, the air you’re breathing in is often the air other people are breathing out,” he says.
Since the viruses are inhaled, “it makes sense that proximity to people who may be contagious would facilitate transmission,” Shaman says.
But there’s more to the story, says Benjamin Bleier, a sinus and nose specialist at Harvard Medical School.
“In today’s society, we are at home all year round,” he says.
To drive the seasonality we see from year to year, something else must be happening that makes people more susceptible to infection and increases the amount of circulating virus, he says.
Dry air can make some viruses stronger.
Some viruses thrive in winter. But the reason may not be so much temperature as humidity.
“There are some viruses that like heat and humidity, and some that like dryness and cold,” says Donald Milton, an aerobiologist at the University of Maryland School of Public Health in College Park. For example, rhinoviruses—one of the many types of viruses that cause the common cold—survive better in moist environments. Rhinovirus cases usually peak in the early fall, he says.
Marr and other researchers found that viruses that grow stronger in winter, including influenza viruses and SARS-CoV-2 — the coronavirus that causes COVID-19 — survive best when the relative humidity drops below about 40 percent.
Viruses don’t usually float around naked, Marr says. They are covered with droplets of liquid, such as saliva. These drops also contain pieces of mucus, proteins, salt and other substances. These other components can determine whether the virus survives drying.
When the humidity is higher, the drops dry slowly. That’s it slow drying kills such viruses , like influenza A and SARS-CoV-2, Marr and colleagues reported July 27 in a preprint on bioRxiv.org. During slow drying, salt and other substances that can harm the virus become more concentrated, although researchers still do not fully understand what happens at the molecular level to inactivate the virus.
But rapid drying in dry air preserves these viruses. “If the air is very dry, the water evaporates quickly. Everything has dried up and it’s almost like things are frozen in place,” says Marr.
How humidity affects air-droplet processes
At low humidity levels, air droplets or aerosols dry quickly (left), trapping the viruses under a feathery crystal lattice, as shown in this microscope image. At medium humidity, crystals form inside the liquid droplets (in the middle), but these crystals can inactivate viruses rather than preserve them. At high humidity (right), aerosols remain liquid, which allows viruses to survive better than at medium humidity.
Dry, smaller aerosols are also more buoyant and can hang in the air longer, making it more likely that someone will inhale them, Fisman says.
Moreover, dry air can weaken a person’s defense against viruses. Animal studies show that dry air can cause the death of some cells , lining the respiratory tract. This can leave cracks where viruses can enter.
Mucus in the respiratory tract can trap viruses and help protect against infection. But breathing in cold, dry air can also slow down the system that normally removes mucus from the body. This can give viruses time to break free from the mucus trap and invade cells, Fisman says.
The cold can damage our ability to fight viruses.
The cold may not cause a cold, but it can make you more susceptible to colds.
Bleier and his colleagues recently discovered that normally the immune system has a trick to defend against viruses. Cells in the nose and other parts of the body are studded with surface proteins that can detect viruses. When one of these sensor proteins sees a virus coming, it signals the cell to release tiny bubbles called extracellular vesicles.
According to Blier, the bubbles work as a distraction tactic, similar to the chaff being released from a military jet trying to avoid a heat-guided missile. Viruses can chase vesicles instead of infecting cells.
If a virus comes into contact with one of the bubbles, it’s in for a surprise: inside the vesicles are bits of virus-killing RNA called miRNAs. One of these microRNAs, known as miR-17, can kill two types of rhinoviruses and the coronavirus that causes the common cold, the team reported Dec. 6 in Journal of Allergy and Clinical Immunology .
How cold weather affects the immune system
The immune system uses a diversionary tactic to prevent viruses from infecting cells in the nose: When viruses (black and gray spheres) are detected, nasal cells release bubbles called extracellular vesicles (blue circles). These bubbles are studded with proteins (red, blue, and black shapes on blue circles) that are normally found on the surface of nasal cells. Viruses can chase bubbles instead of infecting cells. When the temperature in the nose drops below body temperature (right), cells release fewer bubbles, making it easier for viruses to find and infect nasal cells.
The researchers measured the bubbles released from human nasal cells grown in laboratory dishes at 37° Celsius, our typical body temperature. Then the scientists lowered the temperature of the thermostat to 32°C. The team found that cells release about 42 percent fewer vesicles at a lower temperature. Moreover, these vesicles carried fewer weapons. Vesicles can package about 24 percent more miRNAs at body temperature than at cold temperatures.
Three tips to strengthen our immune system.
I asked the experts what people can do to protect themselves from viruses in the winter. Some say that using a humidifier can help increase moisture levels enough to slow down the drying of the virus-laden droplets, killing the viruses.
“Any increase in humidity should be beneficial,” Shaman says. “You get a lot of bang for your buck if you go from very dry to dry.”
But Milton doesn’t think it’s a good idea to pump a lot of moisture into a house when it’s cold outside. “That moisture will find all the cold places in your house and condense there,” creating a breeding ground for mold and rot, he says.
Instead, he advocates turning on exhaust fans in the kitchen and bathroom to improve ventilation and use HEPA filters or Corsi-Rosenthal boxes to filter unwanted viruses from the air.
Blair suggests wearing a mask. Not only can the masks filter out viruses, but “our work shows that these masks have a second mechanism of action,” he says. “They keep the pillow warm [вологого] air in front of our noses, which can help strengthen the immune system.”
