How Fast and Far a Sneeze Can Carry Contagious Germs

What you need to know about SARS-CoV-2

Coughing & Sneezing

“Bless you.”

This simple saying is often heard after someone sneezes.

For many people it’s a common courtesy that’s become so engrained in them it’s an automatic reaction the moment they hear a sneeze.

However, according to new study, when a person sneezes near you, your first priority should be to back away before you offer any blessings.

Why?

Because contagious germs can spread further and faster than you may think.

Researchers at the University of Bristol assessed the airborne survival of bacteria in aerosol droplets from coughs and sneezes.

They found the average sneeze or cough can send around 100,000 contagious germs into the air at speeds up to 100 miles per hour.

These germs can carry viruses, such as influenza, respiratory syncytial virus (RSV) and adenoviruses, which cause the common cold.

They can also carry bacteria, such as Streptococcus pneumoniae or Haemophilus influenzae.

The most critical time for spread of those germs, according to the researchers, is in the first few minutes after a sneeze or cough occurs.

“This type of transmission is of special importance since it doesn’t require proximity between individuals. The droplets’ small size adds the potential to penetrate deeper in the lung,” Allen Haddrell, PhD, one of the study’s authors, told Healthline.

While aerosols that carry the germs eventually drop to the ground, that takes time.

“Given the small size of bioaerosol droplets (diameter less than the width of a human hair), they can remain suspended in the air for prolonged periods of time, from seconds to weeks,” said Haddrell.

How Fast and Far a Sneeze Can Carry Contagious Germs

Most people understand coughing and sneezing can spread germs that cause illness, but the speed and distance they can travel might surprise you. High-speed imaging has helped MIT researchers determine that some droplets from coughs and sneezes may carry much farther than previous studies had estimated.

The important thing to understand here is that scientists really only have estimates for how far coughing and sneezing can spread germs, not hard numbers. Some of this might even depend on how forcefully a person coughs or sneezes. (Scream sneezers, we’re looking at you. But we also know it’s not your fault.)

Large respiratory droplets containing pathogens like influenza can travel up to 6 feet when a sick person coughs or sneezes, according to the CDC. A 2014 study by MIT scientists published in the Journal of Fluid Mechanics suggests this number may be way higher for smaller airborne particles. Researchers used high-speed video upwards of 1,000 frames per second to record sprays of mist as well as human coughs and sneezes, finding that smaller droplet particles traveled as far as 2.5 meters horizontally through the air. That’s more than 8 feet.

The study also recorded smaller airborne droplets spraying 13 to 20 feet vertically in the air, which researchers noted was theoretically high enough to enter and travel through some ceiling ventilation systems in some buildings. The researchers posit that this impressive (and kind of nauseating) distance is because smaller pathogens can travel as part of a buoyant cloud that extends their reach.

The problem with airborne pathogens isn’t just how far they can spread, it’s also how long they can hang out in the air and on objects. A lot of this depends on the pathogen in question. Measles, for instance, can live for up to two hours in the air and on surfaces, according to the CDC. This illness is so contagious that 90 percent of people who are close to a person with measles but who aren’t immune (like through vaccinations) will catch the illness. That’s especially scary considering the recent measles resurgence happening in some parts of the United States.

Unfortunately, even the best cough and sneeze etiquette can’t fully stop the spread of disease, Dr. Roach explains. A small 2013 study of 31 people published in BMC Public Health found that some droplets—especially smaller ones—still spread when the participants were practicing good cough etiquette, including coughing into their shirt sleeve or elbow.

As the scientists explained, this is because some particles manage to find the path of least resistance around whatever is blocking them. But pure physics dictates that putting an obstacle in the way of any pathogens is preferable to just spewing them into the air without any barriers. Even though covering your nose and mouth isn’t foolproof, it’s definitely better than nothing—which is precisely why the CDC recommends it.

In addition to following proper sneeze and cough etiquette, you should wash your hands thoroughly and frequently when you’re sick. (Especially if you slip up and cough or sneeze into your hands.) It’s also kind to try to keep your distance from people when you’re ill, including staying home from work if you can when you’re really sick, and to frequently disinfect surfaces you’re always touching.

References

• Bourouiba L, Dehandschoewercker E, Bush J W M. Violent expiratory events: on coughing and sneezing. Journal of Fluid Mechanics, 2014, 745: 537-563.
• Tang J W, Liebner T J, Craven B A, et al. A schlieren optical study of the human cough with and without wearing masks for aerosol infection control. Journal of the Royal Society Interface, 2009, 6(suppl_6): S727-S736.
• Pennisi, E. Water’s Tough Skin. Science, 343: 1194–1197 (2014). DOI: 10.1126/science.343.6176.1194
• Corie Lok. The snot-spattered experiments that show how far sneezes really spread. Nature 534:24–26, 2016. DOI: 10.1038/534024a
• Zhao, L., Qi, Y., Luzzatto-Fegiz, P., Cui, Y. & Zhu, Y. COVID-19: Effects of Environmental Conditions on the Propagation of Respiratory Droplets. Nano Lett. 20, 7744–7750 (2020).