Interesting facts

Information about asthma and FeNO

How to recognize asthma? What is allergic asthma? What does FeNO stand for?

You can find answers to these and other questions here. Learn more about the causes of asthma, diagnosis, and therapy options, and the prevalence of asthma. Learn how our respiratory system works in general. Also discover how tiny NO molecules in the exhaled breath show whether your lungs are inflamed or not.

Our respiratory tract

Respiratory system

To have a better understanding of what happens in your body if you are an asthma sufferer, it is important to learn how our respiratory system works.

Our respiratory system consists of the upper and lower airways. It supplies us with vital oxygen (O2) during inhalation and simultaneously releases the waste product carbon dioxide (CO2) during exhalation.

The upper airways

The upper airways include the nose, the paranasal sinuses and the throat (pharynx). Air enters the body through the nose. The mucous membrane of the nose is equipped with fine hairs, the ciliated epithelium. Here our breath is filtered, heated and humidified. Additionally, the nose is our olfactory organ. Particularly when you have a cold, you quickly notice when your nose cannot perform its usual tasks. The nose is connected to the throat via the paranasal sinuses.

The lower airways

The lower airways include the larynx, trachea, lungs and bronchi. The larynx contains the vocal chords and also connects the upper and the lower airways. At the upper edge of the larynx the epiglottis is located. It acts as a switch between the airways and the digestive tract, and ensures that breathed air enters the trachea and chyme enters the esophagus. The trachea is connected to the larynx. It is a flexible tube about 12 cm long that branches into the bronchi. The bronchial tubes are part of the system of tubes in the lungs, which conduct air into and out of the lungs. The bronchial system can be imagined as an upside-down tree. The trachea forms the trunk, from which two thick, short branches, namely the left and the right main bronchus, each branch off into one lung. These in turn branch into smaller and smaller bronchi that fill the entire lung and evolve into microscopically fine air sacs called alveoli, which are the site of gas exchange between air and blood. Around 300 million alveoli supply our body with oxygen.

What happens in the lung of an asthmatic person?


The airways of asthmatics are permanently (chronically) inflamed. Therefore, their bronchial system reacts hypersensitively to certain stimuli. In technical terminology, this is called bronchial hyperresponsiveness. The combination of hypersensitivity and acute irritation as well as persistent inflammation leads via several mechanisms to a narrowing of the bronchi (obstruction), whereby breathing air can no longer flow unhindered. Those affected suffer from symptoms such as

  • coughing
  • wheezing
  • shortness of breath
  • and tightness in the chest

Typically, these symptoms occur in attacks, recede, and then flare up again during the next attack.

What is allergic asthma?


Respiratory inflammation can have different triggers, so asthma is divided into the following three forms:

  • Allergic asthma
  • Non-allergic asthma
  • Mixed forms

Allergic (extrinsic) asthma is an allergic reaction of the body to inherently harmless substances. The body develops an allergy to triggers such as pollen or animal hair.

What is FeNO?

FeNO – an inflammatory marker in our breath

NO molecules

The abbreviation FeNO stands for fractional exhaled nitric oxide (NO).

  • NO develops during allergic inflammation in the respiratory tract.
  • The NO concentration can be measured in exhaled breath.
  • There is a proven relationship between the concentration of NO in exhaled breath and the extent of inflammation in the airways – the higher the NO content, the more severe the inflammation!

What do the results mean?

FeNO results

FeNO results are measured in ppb (parts per billion). As a guide, the values can be interpreted as follows:

  • Normal value: under 25 ppb (children under 20 ppb)
  • Indication of active inflammation: over 50 ppb (children over 35 ppb)

Several factors influence the FeNO results. According to scientific findings1,2, the normal FeNO values, for example, depend on height and gender. We can note that

  • women generally have lower values than men, and
  • children have lower values than adults.

1 ATS/ERS Am J Respir Crit Care Med 2005;171:912-30

2 Horváth et al. Eur Respir J 2017;49:1600965

3 NICE: Guideline Asthma - diagnosis and monitoring (draft 2017 July 4,