An Introduction to the Physics of Cough
You probably might have read or heard someone say: “when you cough, the air travels at an extreme speed in your windpipe.”
What happens physically to your body when you cough?
First of all, let’s get acquainted with the three phases of cough:
- Inspiratory phase – The large breathing muscle at the base of the lungs (diaphragm) contracts, bending downwards into a dome shape; the accessory breathing muscles relax. These muscle movements cause lung pressure to decrease (lower than Earth atmospheric pressure): air from the atmosphere rushes into the lungs, and a deep breath in (inspiration) occurs.
- Compression phase – The closing mechanism on the top of the airway (glottis) seals shut while the diaphragm relaxes and auxiliary respiratory muscles contract. Due to the substantial elastic effect of the relaxing diaphragm forcing it to return to a flat shape, pressure in the lungs increases sharply.
- Expiratory phase – The glottis suddenly opens, causing the air at supra-atmospheric pressure in the lungs to leave the lungs rapidly.
The Extreme Pressures and Speed
During intense coughing, pressures inside the chest cavity may reach over 300 mmHg (about three times the average normal blood pressure), and the air rushes out through the airways at about 800 km/h (85% of the speed of sound, or near transonic).
Due to the extreme cough airflow speeds, coughing is such an explosive process, that when repeated enough, causes trauma to the upper airway (larynx) and lower airway (trachea), which is known as “cough-induced” laryngotracheitis (CILT).
Here is a list of the most common possible negative consequences of the extreme physics of repetitive cough when the body doesn’t get a chance to recover:
- Exhaustion/Musculoskeletal pain – Due to the intense contraction of respiratory muscles during cough.
- Self-consciousness – Coughing can often cause people to direct their attention to their respiratory muscles, distracting them from other activities.
- Insomnia – Coughing often may directly conflict with the need to settle down for sleep.
- Headache/Dizziness – The extreme physics of cough can disturb intracranial pressure, cerebrospinal fluid flow and blood flow.
- Hoarseness/mechanical trauma – the repetitive insults of the upper airway (laryngotracheal domain), caused by the near transonic blasts of air shooting out of the lungs directly damage tissues.
- Inflammatory response – with extended insult, a chronic inflammatory response may establish a feedback mechanism where inflammation (cellular and immune pathways) induces further propagation of cough (more mechanical trauma), which causes further inflammation.
The Surprising Blood Flow
The extreme pressure changes to the chest during cough, almost make the chest cavity function as a second heart: a single cough starting with a deep breath can:
- During the inspiration phase, pull a small amount of blood from the extremities to the thorax.
- During the compression and expiratory phases, push almost 300 mL and then 700 mL of blood out of the chest toward the extremities, respectively.
The extreme pressure and blood flow changes during a deep cough made people wonder whether coughing might help during a heart attack.
If you’re experiencing the symptoms of a heart attack, as illustrated in figure 1, you must immediately call the emergency number for your country. While the help arrives, if you are capable of performing deep voluntary coughs, you may do so as it is potentially beneficial to enhance blood flow to the brain. However, keep in mind that cough alone is not enough to save you; only trained EMS/EMT/ER personnel are trained and have the means to do so.
Coughing as a CPR mechanism must NEVER delay contacting emergency services.
Since coughing might increase pressure inside the head (intracranial pressure), it can cause or worsen a headache. For example, in people with migraines, coughing is often an aggravating or triggering factor.
The Frightening Distance Launched Droplets Can Reach
Is the two-meter social distancing, as mandated by health authorities, enough? The short answer is: “Mostly yes indoors; not outdoors.” An average human cough may travel up to unexpected considerable distances depending on the wind speed.
If the wind speed is approximately zero, saliva droplets (and any infectious disease pathogen potentially within them) did not travel 2 m. However, with wind speeds of 4 km/h (slight breeze) to 15 km/h (leaves and small twigs in constant motion, wind extends light flags), the safety level drops considerably since saliva droplets can travel beyond six meters.
Droplets released when coughing (or sneezing) occur in a continuum of sizes and can amount to as much as 7 to 8 mg. These droplets do not travel independently on their trajectories; instead, they are trapped and carried forward within a moist, warm, turbulent cloud of gas.
Properties of droplets released when sneezing or coughing:
- Larger droplets:
- Are formed by saliva.
- Settle more rapidly than smaller ones due to gravitational forces.
- Gradually disperse into smaller ones while moving away from the mouth.
- Smaller droplets:
- Are formed by the mucous coating of the lungs and vocal cords.
- Are often invisible to the naked eye due to their small size.
The research on cough droplet dispersal is clear: no safety measure is too much to avoid infection by a respiratory disease. In public spaces, mask use is fundamental, while distancing recommendations need adjustment for outdoors:
- Indoors in public spaces, a social distance of above two meters is effective.
- Outdoors, if people are not wearing masks, the risk of someone coughing and dispersing droplets beyond six meters is very real. In this situation, permanence near large groups of people is vital to avoid.
Author: Rogério M. – Linkedin
Rogério collaborates with healthcare professionals, artists, and software developers to deliver the best possible science communication content. Background: Biochemistry
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