Health authorities around the world currently recommend that we should ideally keep six feet of space between ourselves and others when out in public to prevent possible transmission of the COVID-19 virus. But what if it’s a particularly windy day?
New research suggests you might want to allow a little more space in these conditions.
A study published in the American Institute of Physics journal Physics of Fluids found that even in relatively mild winds of four to 15 km/h, saliva droplets from coughing or sneezing can travel up to 18 feet “with a decrease in the concentration and liquid droplet size in the wind direction.”
“Our findings imply that, considering the environmental conditions, the [six foot] social distance may not be sufficient,” the authors stated. “Further research is required to quantify the influence of parameters such as the environment’s relative humidity and temperature, among others.”
It’s important to note that the study used computational modeling to simulate weather conditions and how each drop of saliva from someone coughing moved through the air, rather than real-world tests.
Effects of wind and other weather on transmission not well-studied
The study authors also highlight that exactly how airborne viruses are transmitted in general is not well understood. Other researchers have highlighted the lack of studies focused on the impact of wind in the spread of infectious disease.
A 2018 Correspondence article in The Lancet Planetary Health says that “despite being relatively neglected and mainly considered in terms of its direct effects on pathogen dispersion, wind's capacity to carry and disperse signal molecules (e.g., CO2), or other as yet unidentified features, make it a more important factor in the environmental component of the One Health concept.”
Relating to the spread of SARS-CoV-2, the virus that causes COVID-19, early studies have focused mostly on weather and climate in general, rather than wind specifically.
“A single weather factor alone could not affect the virus transmission too much,” say the authors of a preprint study on why weather factors are considered collectively rather than individually when it comes to viral transmission.
In a similar study, researchers focused on how four weather factors, along with population, impacted the number of COVID-19 cases in nine Turkish cities. They found that the higher the average wind speed over the two weeks before each case was identified, the higher the number of cases. The authors note that their study was limited by the fact that not all past meteorological data were available for each city, only nine cities were studied, and they did not know the exact number of people coming into each city, due to visitors being quarantined.
As pointed out in a document by Canada’s National Collaborating Centre for Environmental Health, most studies on COVID-19 viral transmission also focus on indoor, rather than outdoor spaces, making it hard to determine whether public health measures for these spaces are too harsh or not strict enough.
Others state that it’s important to not draw definitive conclusions from the early research that does exist, but some trends are emerging.
According to a review of these studies by the UK’s Centre for Evidence-Based Medicine, SARS-CoV-2 spread appears to be affected by weather “with cold and dry conditions appearing to boost the spread.” However, the center stresses that the overall effect of weather is small and current research still too inconclusive for people to stop following the strict public health measures governments have put in place, even once warmer weather arrives.
A large, susceptible population still the biggest factor
Similarly, a recently published study points out that, because many people still remain susceptible to the virus no matter what climate they live in, local climate is unlikely to have a big impact on how quickly the virus is transmitted.
"It doesn't seem that climate is regulating spread right now," said first author Rachel Baker, a postdoctoral research associate in the Princeton Environmental Institute, in a press release on the study. "Of course, we do not yet directly know how temperature and humidity influence the virus' transmission, but we think it is unlikely that these factors could completely halt transmission based on what we see among other viruses."
The authors of that study do note that it’s possible the new coronavirus, like its cousin the common cold, could eventually “settle down” to become a more seasonal illness once enough people build an immune response.
It will be interesting to see how public health policies change as more research is done in the real world on how SARS-CoV-2 is affected by outdoor environments. In the meantime, keeping as much space as possible between yourself and others is likely the best course of action when you’re out on windy days.