By Deborah Baines, St. George’s University, London, Trustee of The Physiological Society
There is misinformation currently circulating regarding face masks reducing the oxygen level in our blood. I am here to dispel this concern by explaining the physiology of oxygen flow in your lungs and blood.
Paper face masks and fabric face coverings are permeable to air but restrict the movement of water droplets that we exhale or sneeze. The two key gases for respiration, oxygen and carbon dioxide, can move freely across these materials.
During normal breathing we inhale about 500 ml per breath. As we take approximately 12 breaths per minute, this amounts to about 6000 ml (6 litres) of air inhaled per minute. Not all the air, however, reaches the tiny sacs at the tips of the branching structures in our lungs, called the alveoli. This is where gas exchange with our blood takes place.
The air that stays in the conducting airways (trachea and bronchi) and does not make it to the site of gas exchange is called “anatomical dead space.” This is about 150 ml for each 500 ml breath. In other words, with each breath, only about 350 ml is available for gas exchange, meaning this is the amount that could make it to our tissues and help keep them healthy.
Theory governing gas transport in the body uses the partial pressure of oxygen in the atmosphere (the amount of oxygen compared to other gases), because this determines how much oxygen is available for our bodies.
The partial pressure of oxygen in the air is approximately 21 kPa if barometric pressure is 100 kPa which is usually the case around sea level. In other words, 21% of the air we breathe is oxygen, when we are not at high altitude. When we are at high altitude, this changes, as atmospheric pressure and thus oxygen partial pressure decrease.
Thus, 882 ml of oxygen per minute gets to the alveoli (350 ml x 12 breaths x 21%).
Our bodies generally use 250 ml of this oxygen per minute. What is left then mixes with the carbon dioxide (that is being released for exhalation) and water vapour, so the amount of oxygen at the alveoli (in this example) is approximately 15% (882- 250/ 350 x 12).
Oxygen in the blood equilibrates with oxygen in our alveoli. Blood is maximally saturated with oxygen when alveolar oxygen is 13-15%. This means this means that all the all the oxygen binding sites on haemoglobin (which makes our blood red) are occupied. So even if more oxygen were available, blood saturation with oxygen does not increase. Furthermore, the relationship between % oxygen and blood saturation is not linear, oxygen levels would need to halve to reduce saturation to 90%.
Increased activity requires more oxygen and so you breathe deeper and quicker, to maintain the saturation of your blood with oxygen.
What might a face mask do? There has been discussion that a face mask might add to anatomical dead space by extending it beyond the mouth area. If everything else stays the same, an extra 20 ml say, of dead space, theoretically will reduce the percentage of oxygen at the alveolus, but the effect is very small, meaning that blood still would stay fully saturated with oxygen.
Think about how many people go swimming with snorkels, how much dead space this adds and how it has remarkably little effect on our activities.
So, does a face mask restrict flow of air into the lungs? The bottom line is, if used correctly, it does not. If airflow is restricted, less millilitres of oxygen get to the alveoli, and less carbon dioxide is exhaled. While this reduces the percentage of oxygen in our lungs, and increases carbon dioxide, the body senses these changes in the lungs and stimulates breathing.
This means that you will take more breaths and blood oxygenation/saturation will be maintained. In other words, paper face masks and fabric face coverings do not affect blood oxygen saturation, so please spread the word and counter misinformation you see on the internet or hear in conversations.