We’re told time and time again to cover our coughs and sneezes, especially when we have a cold or the flu. We’re led to believe that the tiny droplets that rush out of our noses and mouths could spread the virus to others nearby ⁽¹⁾.

But just how well can flu viruses travel through the air? And what’s the risk of us spreading them via the sprays excreted through our noses and mouths? To answer these questions, you first need to grasp the dynamics of our coughs and sneezes.

Why we cough and sneeze

Coughs and sneezes have one simple goal in mind: ridding our bodies of irritants, foreign bodies and bugs, including viruses ⁽²⁾.

Coughs ⁽²⁾ are a reflex that aims to clear out our airways quickly. A deep breath in is followed by a forced breath out. The sudden release of air as our vocal cords open up creates the very distinctive sound we’re all familiar with. Sneezes ⁽²⁾, on the other hand, are a coordinated, yet involuntary, effort by multiple parts of our bodies to clear our nasal cavities whenever something is irritating them.

A cough can travel as fast as 80 km/h while a sneeze can reach up to 160 km/h ⁽²⁾. Unfortunately, these rather violent eruptions expel a cocktail of saliva, mucus and irritants within their droplets of moisture. A cough can expel almost 3,000 droplets while a sneeze can expel upwards of 100,000 ⁽²⁾. When we have the flu, these will almost certainly also contain some of the viruses caught in our airways.

Flu is in the air

Most experts believe the droplets made when a person with the flu coughs or sneezes are the main way flu viruses spread ⁽¹⁾. Following a cough or sneeze, the droplets might land in the mouths or noses of people nearby, or someone could breathe them in ⁽¹⁾. Whichever way they enter that person’s body, that person could catch the flu.

The droplets typically reach about one metre before hanging suspended in the air for a while ⁽³⁾, where they can survive as droplets in the air for several hours. After that time, they may land on surfaces, where they can survive for up to 24 hours ⁽³⁾⁽⁴⁾. Everyday items at home and in public places can easily become contaminated, including food, door handles, remote controls, handrails, telephone handsets and computer keyboards ⁽³⁾. People can catch the virus by touching the surfaces droplets have landed on if they pick up the virus on their hands and then touch their nose or mouth ⁽¹⁾⁽³⁾.

Recent studies have also found large amounts of the flu virus in the breath of people with the flu ⁽⁵⁾. This means the flu could contaminate the air around a sick person just by them breathing, without them even coughing or sneezing.

Winter helps the flu spread

Low temperatures increase how long the flu virus can survive in the air ⁽⁴⁾, which explains why the flu is more common during the winter ⁽⁶⁾.

Research has shown that the flu virus’ ability to survive is linked to the air’s ‘absolute humidity’ ⁽⁶⁾⁽⁷⁾ – or the percentage of water vapour in the air. Air with a lower absolute humidity is drier. So, while the weather outside may seem wetter during the winter months, the laws of thermodynamics mean cold air can carry less water vapour ⁽⁶⁾; in other words, it has a lower absolute humidity – is drier.

In moist air, the droplets we expel when we cough or sneeze remain relatively large, while in dry air they break up into smaller pieces ⁽⁶⁾. The larger droplets fall out of the air and onto surfaces more quickly than the smaller pieces, which may stay afloat for hours. The result is that, in winter, we are more likely to be breathing in the droplets containing a cocktail of saliva, mucus, irritants and viruses ⁽⁶⁾. We’re more likely to catch the flu.

Protecting yourself and your loved ones

Flu viruses can survive as droplets in the air for hours and on surfaces for up to a day but can only survive on tissues for 15 minutes ⁽⁴⁾. Bearing this in mind, if you do catch the flu:

• Use tissues to cover your mouth and nose when you cough or sneeze. Put them in the bin straight away ⁽³⁾.
• Alternatively, cover your cough or sneeze with your sleeve in the bend of your arm. Never use your hands ⁽²⁾.

If anyone in your home has the flu:

• Wash your hands regularly for 20 seconds with soap and water ⁽³⁾.
• Disinfect surfaces such as door handles, remote controls and telephones ⁽³⁾.
• Avoid touching your eyes, nose or mouth ⁽¹⁾.

While these steps can give you some protection from the flu, getting a seasonal flu vaccine every year is the most effective way to prevent the flu ⁽⁸⁾. If you’re unfortunate enough to become infected, a flu shot will make your symptoms milder ⁽⁹⁾. Have you had yours?

References
(1) https://www.cdc.gov/flu/about/disease/spread.htm
(2) https://www.lung.org/about-us/blog/2016/05/sneeze-versus-cough.html
(3) https://www.nhsinform.scot/illnesses-and-conditions/infections-and-poisoning/flu
(4) https://www.nhs.uk/common-health-questions/infections/how-long-do-bacteria-and-viruses-live-outside-the-body/
(5) https://sph.umd.edu/news-item/flu-may-be-spread-just-breathing-new-study-shows-coughing-and-sneezing-not-required
(6) http://www.bbc.com/future/story/20151016-the-real-reason-germs-spread-in-the-winter
(7) https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1000316
(8) https://www.cdc.gov/flu/prevent/index.html
(9) https://www.fda.gov/forconsumers/ucm384535.htm