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Mouth Breathing versus Nostril Breath

Years ago, when I was first taught about the science of breath by an enthused and slightly eccentric yoga teacher, I was told that 'animals breathe through their mouths just before they die' and thus to preserve life, breathing through the nostrils should be favoured at all times. Like a good student, I swallowed this notion whole without wondering why it might be true – until I was asked about it recently, encouraging me to do some research.

According to published clinical evidence, mouth breathing is a leading cause of mortality for those suffering chronic conditions. So it would seem that it is not only animals who breathe their mouths before die! But why is this? Why is mouth breathing so bad?

Here is what I discovered:


Our nasal passages are covered with a thin layer of protective mucus designed to humidify, cleanse and warm incoming air. This mucus traps roughly 98% of bacteria, viruses, dust particles and other airborne objects, moving them in a steady stream through various bodily pathways to the stomach, where acids and enzymes either kill or weaken them. These dead or weakened cells are then absorbed through the lining of the digestive tract, giving feedback to our immune system, allowing it to learn how to protect itself against them.

Breathing through the mouth bypasses this process completely, encouraging pathogens to settle and reproduce all over the body, causing infection and unnecessary allergic response.


Oxygen (O2) is vital to the correct functioning of the body - without it, we cannot survive. But we can have too much of a good thing. Contrary to popular belief, carbon dioxide (CO2) is not a toxic waste gas, but an important regulator of O2 absorption in the cells. Perhaps surprisingly, we have a constant store of it in what biologists describe as 'dead space' inside the nose, throat and bronchi. During nasal inhalation, some of this stored CO2 is drawn into the respiratory system alongside the fresh oxygen and then used to create the appropriate balance of O2 and CO2 in the cells (known as the Bohr Effect).

Breathing through the mouth disturbs this delicate balance of gases as it brings too much pure oxygen into the system, whilst reducing levels of carbon dioxide. This has the effect of constricting blood vessels, raising blood pressure, creating anxiety and tensing muscles. Drawing cold air straight into the system also contributes to significant biomechanical stress.

Hormone and Heart Health

As well as being lined with a protective mucus, our sinuses generate nitric oxide (NO), a key player in heart and hormone health. Not only does NO contribute to the destruction of viruses, parasitic organisms and malignant cells in the airways, but it also regulates vasodilation (thus lowering blood pressure), inhibits inflammation and influences the secretion of a number of hormones, particularly adrenaline.

Breathing through the mouth has no activating effect on nitric oxide, robbing the body of many of its health-giving properties.


The dictionary definition of a mouth-breather is a 'stupid person' (check it out if you don't believe me!) This is likely to be because nitric oxide is essential for neurotransmission, or cell communication with the brain. Memory, sleep, learning, the sensation of pain and many other processes are possible only with NO present. Given that mouth breathing offers no access to nitric oxide, breathing through the nostrils seems like a pretty good idea if we want our brains to function optimally.


Interestingly, whilst we have seen that breathing in through the mouth isn't good, breathing out through the mouth – or sighing, is actually a crucial autonomic reflex required to keep the lungs in good working order. Scientists have discovered that, unless we breath deeply every 5 mins or so, our alveoli (the tiny sacs responsible for the transfer of oxygen and carbon dioxide from the lungs to the blood stream) collapse. If they are not re-inflated, lung failure occurs. And so the brain periodically sends us the signal to sigh, ensuring that the body stays sufficiently oxygenated.

Sighing also has an emotional component, as it tends to occur when we are frustrated, negative or bored. Studies suggest that when the brain gets stuck in someway, particularly when faced with problems, the body steps in to lend a helping hand by triggering the deep breath of a sigh in order to reboot mental functioning and promote clarity.

Being Breath Aware

Bearing all this in mind and bringing it consciously onto our yoga mats, a particularly potent exercise to begin practice would be to breathe in deeply through the nose and then to sigh out of the mouth 3 or 4 times. This combines the cleansing, health-giving, mentally stimulating properties of nasal breathing with the 're-set' function of sighing. To breathe in this way leaves us alert and ready, better able to engage in mindful activity.

Off the mat, it is clearly advisable to be conscious of breath not simply as a way of staying present (as is often taught), but also as a way of staying healthy, particularly in public places where the potential for infection and stressful over-stimulation is more acute.


First published in 2018

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