The Knife Test: A Simple Way to Check Nasal Patency

Many people breathe through their mouths without realizing it. One common reason is reduced nasal patency, meaning the nose is partially blocked or airflow through the nostrils is uneven. An easy at-home way to get a general sense of nasal airflow is something called the Knife Test.

What Is the Knife Test?

The Knife Test is a basic screening tool used to observe airflow coming from each nostril. It can help identify whether one side of the nose may be more congested or restricted than the other.

To perform the test, you hold a smooth metal object such as a butter knife, spoon, or metal ruler horizontally underneath your nostrils and gently breathe out through your nose. As you exhale, condensation forms on the metal surface. The size and shape of the fog marks can give you a rough idea of airflow from each nostril. 

If our nose is chronically blocked, we are likely to switch breathing through our mouth. Learn why nose breathing is so important, and why mouth breathing directly impacts our overall health.

How to Perform the Knife Test

1. Find a clean metal spoon or butter knife.
2. Hold it horizontally just under your nostrils.
3. Take a gentle breath in through your nose.
4. Slowly exhale through your nose onto the metal surface.
5. Observe the condensation pattern.

Ideally, you will see relatively equal fogging from both nostrils. If one side consistently produces much less condensation, it may suggest reduced airflow on that side. 

Why Nasal Patency Matters

Good nasal airflow is important for efficient breathing. The nose helps:

• Filter and humidify air
• Produce nitric oxide
• Support diaphragm function
• Improve oxygen exchange
• Encourage calmer, slower breathing patterns

When nasal airflow is restricted, people are more likely to default to mouth breathing, especially during sleep or exercise.

What Can Affect Nasal Airflow?

Reduced nasal patency may be influenced by:

• Allergies
• Congestion or colds
• Deviated septum
• Swollen nasal tissues
• Chronic mouth breathing habits
• Environmental irritants

It is also normal for airflow to naturally alternate somewhat between nostrils throughout the day due to the nasal cycle.

What to Do If You Notice Reduced Airflow

If the Knife Test consistently shows poor airflow on one or both sides, some strategies that may help include:

• Nasal saline rinses
• Steam or humidification
• Allergy management
• Breathing exercises
• Improving posture and rib cage mobility

If symptoms persist, an assessment with a healthcare professional may be appropriate to evaluate for structural or respiratory concerns.

At Move Authentically Physiotherapy, we assess breathing patterns and nasal breathing function as part of a whole-body approach to health and movement. If you are in Kingston and curious about how your breathing may be affecting your sleep, performance, tension, or overall wellbeing, we would be happy to help. Book with us here.