The disgusting reason why air conditioners smell so bad? DEAD SKIN: Bacteria in AC units that feed on our cells produce a nasty ammonia scent

There’s a disgusting reason why air conditioning units often smell so bad.

Scientists have discovered that the stench is caused by dead skins cells from our bodies that are trapped in the units.

The bacteria from the skin cells produce pure ammonium, a smelly chemical that makes up urea – the main ingredient in urine.

The study was led by Dr Lai Ka-man, an associate professor at Hong Kong Baptist University.

Dr Lai Ka-man and her team examined seven bacterial species and four fungal species commonly found in air conditioning units.

They found that ‘skin squames’ – dead skin cells that have peeled off the surface – are a source of food for the bacteria that live in AC units.

Dr Lai said: ‘For effective long-term odour control, it is important to reduce the amount of skin squames from entering the air-cooling units.

‘The simplest way is to install an appropriate filter to capture the skin squames in the air.

‘A filter that can effectively capture particles less than [0.001 centimetres] should help improve the odour problem.’

Dead cells contain keratin, a structural protein which makes up fingernails and hair in humans.

In the animal kingdom, keratin is the material that makes up the valuable horn of a rhinoceros.

The scientists say that the keratin found in human skin cells are broken down by enzymes called keratinases.

When the keratinase chemicals digest the skin cells, the keratin is broken down and produces ammonium.

As it passes through the liver, ammonia is converted to ammonium and then to urea – the main ingredient in urine.

Under severe dehydration, the urine of a person can become highly concentrated and the scent of ammonium strengthens.



Ammonium is a chemical made of nitrogen and hydrogen (NH3).

Proteins are made of building blocks called amino acids, which are also made from nitrogen and hydrogen.

Proteins make up a large portion of the human diet and are converted to ammonium after they are broken down in the liver.

Because ammonium is toxic to humans, the body converts it to urea – the main ingredient of urine.

Under severe dehydration, the urine of a person can become highly concentrated and the scent of ammonia strengthens.

In air conditioning units, the bacteria that live there contain chemicals called keratinases which break down the proteins in the skin cells.

As a result, these bacteria produce pure ammonium which is circulated by the air conditioning unit – spreading the familiar smell of urine.

This means that when bacteria in air conditioning units pump out ammonium as they break down human skin, they produce a urine-like smell.

Whilst keratin produces a urine-like smell, other chemicals found in human skin cells, – lipids and triglycerides, for example – can create other foul-smelling odours.

Dr Lai said that since many Hong Kong people spend more than 70 per cent of their time indoors, air quality should be given due attention.

The study was published in the International Journal of Indoor Environment and Health.

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