More colds in cold weather

Flu virus transmission is dependent on specific temperatures and humidity levels

They don’t call it “seasonal flu” for nothing: the influenza virus is more common in the winter months than in summer time. The question is, why?

Over the years, many answers have been proposed. For example, it has been suggested that cold weather affects behaviour, making virus transmission between people easier. The logic works like this: when it’s cold outside, people tend to spend more time indoors with other people, increasing the chances of transmission. In the summer, people spend more time outdoors and are generally more socially distant from others, making transmission less likely.

Other theories range from a lack of sunlight in winter months lowering vitamin D and making the immune system more vulnerable, to fluctuation in the movement of the upper atmosphere.

With so many possible conditions affecting the spread of flu, pinning the reason down with evidence has been difficult. A new study, however, seems to make a definitive link between temperature and humidity, and virus transmission. At least for guinea pigs.

Temperature and humidity

Researchers at the Mount Sinai Medical School in New York City put caged, un-infected guinea pigs downwind from infected guinea pigs in cages. The scientists then began to change the humidity and temperature in the air over time. They then tested to see under what conditions flu transmission occurred the most.

Results showed that at room temperature, the flu virus spreads most frequently at a humidity of 20% to 35%, which is very dry air. Interestingly, a humidity of 65% was also good for transmission. At about 50% humidity, transmission was less likely to occur, and at 80% humidity, transmission did not occur at all.

The research also revealed that at relatively cold temperatures, about 5 degrees Celsius, transmission was much more likely no matter what the humidity. However, at 30 degrees Celsius, no transmission occurred.

"Guinea pig" guinea pigs

Although the phrase "guinea pig" is now synonymous with "experimental subject," these days specially bred rats or rhesus monkeys are more likely to be experimental subjects. However, the use of actual guinea pigs in the laboratories of the world used to be common.

For this study, scientists were looking to find an animal that has a respiratory system that functions as near to a human’s as possible. This way, the results found for the animals were also more likely to be true for humans. Many animals, including most typical lab animals, do not get human type influenza viruses. However, reading through the literature about the Spanish pandemic influenza of 1918-1919, the researchers came across a paper noting that the flu had affected guinea pigs. Therefore, for this type of study, guinea pigs make for decent, well, guinea pigs.

The use of guinea pigs had an added bonus: when the animals are infected with the flu, they do not cough or sneeze, reducing the chance of spraying virus particles into the air. Since the animals had no direct contact, the researchers concluded that the virus was transmitted through the air simply by the animals breathing.

Nasal mucus and membranes, viral stability and, water droplets: the host, the invading organism, and the local transportation system

So what is it about certain humidity levels and temperatures that make flu transmission more likely? The authors of the study suggest three ways that might have an influence.

Firstly, breathing dry air may slightly damage mucous membranes in the nose, making it easier for the virus to get inside one’s body. Additionally, colder temperatures have an effect on the viscosity of nasal mucus; in effect, the colder it is, the less runny a person’s snot is. In turn, this makes getting the virus out of the upper respiratory tract more difficult.

Secondly, the researchers note that other studies have shown that the influenza virus is more stable at certain humidities, which closely match the preferred humidity levels for transmission. In other words, the virus can live longer in the environment at these humidity levels, increasing the chances of successful transmission.

Lastly, the researchers noted that water droplets, on which the virus is catching a ride, tend to fall right out of the air at high humidity levels: as more water molecules attach themselves to the droplet, they become too heavy and fall to the floor. Conversely, in dry air the droplets remain small and stand a better chance of floating in the air, carrying the virus from one place to another.

Limits of the study

So, to protect the rest of the family, should you break out all the humidifiers and crank up the heat when your child is sick at home with the flu? Maybe, but it should be noted that the study, while instructive, does have its limits. Probably the biggest limiting factor of all is the fact that this research was done on guinea pigs. As similar as their respiratory systems may be to those of humans, they are not the same; nor is their behaviour.

Another factor is that there are many different types of influenza viruses circulating at any given time. While this research may hold for some of these flu types, for other flu types, the preferred temperature and humidity levels for transmission may be entirely different.