Sneezing

Children are famous for runny noses, sneezing and coughing.  This happens for many reasons not the least is that they are building up their own immunity.  Small children can have 8-12 colds a year.

The respiratory tract is very good at cleaning the air that is inhaled.  The average urban dweller inhales billions of foreign particles everyday such as dust, pollutants, pollen, fungal spores and viruses.  If the particle is big enough or irritating enough (pepper), it causes a cough or sneeze to push it back out.  If too small to provoke a sneeze, it is trapped in the mucus of the nasal passage or bronchial tubules in the lung. These tiny airways are lined with millions of hair-like cilia that act like tiny paddles that push the particles up to the back of the throat were the mucous is swallowed and goes to the stomach to be destroyed by the stomach acid. Unfortunately some don’t get pushed out and pass through to make us sick.  Some stay in the lung and infect the lung cells.

Professor Lydia Bourouiba, a mathematician, used high-speed video running at thousands of frames per second to capture a sneeze and then she could play it back in slow motion allowing measurements of the size and shape of the cloud and the speed and distance the particles of the cloud can go.  These videos revealed a violent explosion of saliva and mucus spewing out of the mouth in sheets (like a liquid cling film) that break up into droplets, all suspended in this turbulent cloud.  She found that the sneeze droplets can travel up to 24 feet and can be suspended for 10 minutes.

These studies are helping us understand how some diseases are spread.  It is known that measles is airborne and highly contagious.  Ebola is mainly transmitted by body fluids while SARS was thought to be by close contact with some being airborne.  Covid 19 is now known to be airborne and ongoing studies are being done to determine how infective it is.  Flu viral RNA has been found in particles emitted to the air by just breathing and talking, not violent sneezing.

The complicated sneeze studies have shown that the droplet formation can be influenced by environmental conditions such as humidity and temperature.  Cold weather may influence the sneeze droplet to stick together which may explain why flu and colds are more common in cold weather or is it because more people are in closed spaces unable to “social distance”?

Dr. Bourouiba is working to see if she can develop a mathematical model which could give an airborne score such as this virus is 85% airborne spread and compared to one that is 5% spread.  This knowledge would help public health experts guide the public how fast and how far the outbreak will grow. Understanding the sneeze helps us understand how important it is to wear a mask for Covid 19 as even the simple act of breathing can infect another.

by Sally Robinson, MD 

Clinical Professor of Pediatrics
Keeping Kids Healthy - UTMB Health Pediatrics
Published 7/10/2020