Study shows 1.5m not enough to save you

Respiratory droplets from a cough or sneeze travel farther and last longer in humid, cool climates than in hot, dry ones, according to a new study on droplet physics.

Scientists from the University of California San Diego have created a model that that can predict the early spread of respiratory viruses including COVID-19, and the role of respiratory droplets in that spread.

They now better understand the role that droplet clouds play in the spread of respiratory viruses, including how long the breath of a sick person can affect healthy people and how far the infected droplets can travel.

“The basic fundamental form of a chemical reaction is two molecules are colliding. How frequently they’re colliding will give you how fast the reaction progresses,” said Professor Abhishek Saha, one of the paper’s authors.

“It’s exactly the same here; how frequently healthy people are coming in contact with an infected droplet cloud can be a measure of how fast the disease can spread.”

Depending on weather conditions, some respiratory droplets travel between 2.4m–4m away from their source before evaporating, without accounting for wind.

This means that without masks, the currently mandated 1.5m of social distance may not be enough to keep exhalated particles from reaching someone else.

“Droplet physics are significantly dependent on weather,” said Prof. Saha.

“If you’re in a colder, humid climate, droplets from a sneeze or cough are going to last longer and spread farther than if you’re in a hot, dry climate, where they’ll get evaporated faster. We incorporated these parameters into our model of infection spread; they aren’t included in existing models as far as we can tell.”

The researchers hope their work will help inform public health policies and be used in the future to better understand the role of environmental factors in virus spread.

They also found that at 35°C and 40 per cent relative humidity, a droplet can travel about 2.4m. However, at 5°C and 80 per cent humidity, a droplet can travel up to 3.6m.

The team also found that medium-sized droplets possess higher risk as they take longer to evaporate and travel greater distances. Whereas smaller droplets evaporate within a fraction of a second, while larger droplets quickly settle to the ground due to weight.

The research provides further evidence of the importance of wearing masks in public.

The team is now working to learn more about how COVID-19 droplets spread and how long they live on surfaces.

“Our next step is to relax a few simplifications and to generalise the model by including different modes of transmission,” said co-author Professor Saptarshi Basu.

“A set of experiments are also underway to investigate the respiratory droplets that settle on commonly touched surfaces.”

Do you think this research provides further evidence backing the need to wear masks in public?

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