What does nitric oxide have to do with nasal breathing?

Your sinuses produce a gas called nitric oxide, and it flows into your nasal airway all the time. When you breathe in through your nose, some of it travels down to your lungs, where small studies suggest it may help a little with oxygen uptake. Beyond that, the research is thin, and many of the claims you'll see online go further than the studies do.

Where does nitric oxide in the nose come from?

Nitric oxide is a gaseous messenger the body uses in many ways, including widening blood vessels. It also has antimicrobial activity.

In 1995, researchers at the Karolinska Institute reported in Nature Medicine that the nitric oxide in nasal air is made by the cells lining the paranasal sinuses, the air-filled spaces around your nose. They measured it in sinus air at very high concentrations, close to the highest permissible levels for atmospheric pollution. The lining of the nasal cavity itself showed only weak activity of the enzyme that makes it.

So the source is your sinuses. Your nose is the route the gas takes on its way out, and on its way back in.

What does it do in the sinuses?

Nitric oxide in the sinuses likely helps local defences against germs. The 1995 authors pointed to the known bacteriostatic effects of nitric oxide and suggested it may help keep the sinuses sterile. A 2008 review by Jon Lundberg, the study's lead author, says the concentration in a healthy sinus is higher than what's needed for antibacterial effects in lab tests. He writes that its role is likely to support local defences, by slowing the growth of germs and stimulating the tiny hairs that move mucus along.

That word "likely" matters. It's a reasonable conclusion from lab work and physiology. It hasn't been tested by comparing people who breathe one way with people who breathe the other.

The same review notes that nasal nitric oxide is extremely low in people with primary ciliary dyskinesia and cystic fibrosis, two conditions that come with a high risk of chronic sinusitis. Low nasal nitric oxide is used as a diagnostic clue, especially for primary ciliary dyskinesia.

Does nasal breathing send nitric oxide to your lungs?

Yes, nasal breathing sends sinus nitric oxide to your lungs. A 1996 paper from the same group describes sinus nitric oxide as reaching the lungs with each breath in, especially during nasal breathing. They then tested whether that makes a measurable difference.

In six out of eight healthy volunteers, oxygen tension measured through the skin was 10% higher during periods of nasal breathing than during periods of mouth breathing. They also studied patients on ventilators, whose breathing tubes bypass the nose entirely. In six out of six long-term intubated patients, adding air drawn from the patient's own nose to the ventilator raised arterial oxygen by 18%. In 4 of 12 patients who had been on a ventilator for a shorter time, adding nasal air lowered the resistance in the lungs' blood vessels by 11%.

Lundberg's 2008 review describes nasal nitric oxide as reaching the lungs in diluted form and improving oxygen uptake by widening local blood vessels. He suggests thinking of it as an "aerocrine" messenger, one that's made in the nose and sinuses and carried to where it acts with every breath in.

Does mouth breathing mean you miss out?

Mouth breathing means you miss out on nitric oxide to some degree. The 1996 authors describe sinus nitric oxide as reaching the lungs especially during nasal breathing. If you breathe through your mouth for much of the night, more of each breath skips the nose and you inhale less of it.

For people with a clear nose, nose breathing is the normal state of affairs at night anyway. In a 2003 study of 10 healthy adults with normal nasal resistance, the mouth carried only about 4% of the air they breathed in during sleep, and that share didn't change across sleep stages. So a healthy sleeper is already getting a steady supply of their own nasal nitric oxide all night. The habit to watch for is the slow drift into mouth breathing, often because the nose feels stuffy when you lie down.

What don't these studies show?

The nitric oxide studies don't show long-term effects, because they're small and measured physiology over short periods. The healthy-volunteer part had eight people, and two of them didn't show the effect. Intubated patients in intensive care are a very different group from a healthy adult asleep at home.

None of these studies measured sleep quality, daytime energy or long-term health in people who changed how they breathe.

When a trial has tested a practical way of encouraging nose breathing at night, the result was flat. In a 2009 randomised crossover study, 51 adults with symptomatic asthma taped their mouths at night for four weeks. Taping had no effect on asthma control compared with usual breathing. That's one trial in one condition, but it's a useful check on the idea that more nasal breathing automatically adds up to a measurable benefit.

A fair summary is that nasal breathing delivers sinus nitric oxide to your lungs, and that this may play a small part in how well they take up oxygen. Saying that nasal breathing "boosts nitric oxide" to improve sleep or health goes beyond the evidence.

Does humming raise nitric oxide?

Humming raises the amount of nitric oxide you breathe out through your nose. In a 2002 study of 10 healthy people, nasal nitric oxide was 15 times higher during humming than during quiet exhaling. The researchers' explanation is that the vibrating airflow ventilates the sinuses better, so more of the gas stored there comes out.

The authors suggested this could become a simple test of sinus function, and said any therapeutic effects of humming should be investigated. That's still where it stands. The study measured nitric oxide in single breaths and didn't look at any health outcome.

A follow-up study from the same group in 2003 looked more closely at what happens when you hum. A single hum caused a large peak in nasal nitric oxide, which then tailed off. When people hummed several times in a row, each peak was smaller than the last, but it recovered completely after three minutes of quiet breathing. The size of the openings between the sinuses and the nose mattered most, and the pitch of the hum played a part as well. The authors saw humming mainly as a way to measure how well the sinuses are ventilated.

If you enjoy humming as part of winding down, there's nothing wrong with it. Just treat it as a pleasant habit rather than a treatment, and know that a long string of hums won't keep adding more.

What does this mean for breathing at night?

If you breathe through your mouth all night, you miss out on what your nose does to the air on the way in, which we cover in nasal vs. mouth breathing. Nitric oxide is one part of that picture. Whether it changes how you sleep or feel hasn't been studied well.

The more useful question is why you breathe through your mouth in the first place. A blocked nose and plain habit call for different fixes, so start with why you might breathe through your mouth at night and our two-minute nose test. If your nose is clear and it's habit, some people use mouth tape as a reminder to keep their lips closed. That decision should rest on comfort and safety, not on a promise of more nitric oxide.

Can mouth tape or nasal strips help you keep the nasal route?

Mouth tape and nasal strips can help you keep the nasal route, and that's the realistic way to think about them. We didn't find a study that measured nitric oxide with either one, so neither can claim to raise it. What they can do is make it easier to stay on the route that carries sinus nitric oxide to your lungs.

If the front of your nose feels tight when you lie down, a nasal strip gently lifts the sides of the nose and can make nose breathing feel easier. How nasal strips work explains how. If your nose is clear but your mouth falls open once you're asleep, a strip of mouth tape is a simple reminder to keep your lips together. Our first-week guide walks you through starting, and mouth tape vs. nasal strips helps you choose between them. Either way, check your nose first with the nose test, and skip tape on nights when your nose is blocked. More on the topic is in our nasal breathing articles.

If you want to try mouth tape and nasal strips together, our pick is the Titan Sleep System, which pairs TitanAir Nasal Strips with Titan Mouth Tape.

* This content is for informational and educational purposes only and is not medical advice. It is not intended to diagnose, treat, cure or prevent any disease or health condition. Talk to a qualified healthcare provider about your health.