Air conditioning systems are installed in different buildings for different reasons, but there are two main motives; one is temperature control and the other is air quality.

Around the world, the commonest reason is temperature; tall buildings with lots of glass need temperature control to keep them habitable and homes in hot countries need air conditioners for the same reason. Almost all modern cars also come with air conditioning as standard as well.

Air quality is also a key motivator. In the UK, our factories and commercial buildings are subject to strict air quality regulations and we need our office buildings to provide respite from traffic fumes and allergens.

With air conditioning being such a huge part of everyday life for so many of us, reports and rumours of air conditioner sickness are all the more alarming. Medical investigations have uncovered clear links between air conditioning systems and symptoms such as breathing difficulties, irritated skin as well as headache and fatigue.

So are our conditioners keeping us safe or exposing us to additional risks?


A system that is not designed and maintained properly can actually pump pollutants and irritants into a building along with cool air. In many cases, the problem is just that the air is too dry, giving some people sore throats. In a small number of instances, there are faint “mineral” or “chemical” odours. These are usually due to over stringent cleaning with bactericides or detergents by inadequately trained personnel. They are not dangerous, but they are an irritant nonetheless. Avoiding this nuisance is simple; make sure you use professional engineers for your servicing.


By far the greatest danger with an air conditioner, especially in a large building, is the growth of bacteria, mildew, moulds, yeasts or fungus inside the ducts and other system components. If conditions conducive to microorganism growth are allowed to develop in any one place, the ducts will probably spread them, or the toxins they produce (which include endotoxin8 and beta-1–3-glucans) through the entire building.

Few commercial systems are fitted with air filters capable of removing microorganisms; they are simply too small. Even if you do have antimicrobial filters, they may not be located in the right place to eliminate the hazard when it is the air conditioner itself that is harbouring the problem. Poor quality filters, or those that are not cleaned and replaced when they should be, can become breeding grounds for microorganisms rather than removing them.

Microorganisms in an air conditioner are not always hazardous – after all, every breath of fresh air contains microorganisms and they hardly ever do harm. However, harmful organisms can arise in particular conditions that air conditioning systems may create. When the systems in many older buildings were designed, those conditions were very poorly understood.

When air is drawn over the coils inside an air conditioner there is moisture condensation. If it is not properly channelled away, it provides a stagnant environment for unwanted guests to take root. Some older air-conditioners have condensation pans where water just sits. Listeria is one notorious organism that loves these cold, damp, dark environments.

In other parts of the system, cold dry conditions encourage viruses such as influenza. The preference of cold viruses for cold dry conditions has only recently been proven. Fresh food aisles in supermarkets and other cold storage facilities are often dry and uncomfortably chilled. Research indicates that cold dry air causes blood to retreat from the surface of the skin and throat, making it easier for viruses and other microorganisms to enter.


Professionally installed air conditioners that are properly maintained do not create a microorganism hazard. On the contrary, in providing constantly moving clean air, they eliminate the damp stale corners preferred by moulds and mildews from developing anywhere in your home or building.

Although good design and maintenance make a tremendous difference, some systems will also benefit from the installation of additional technical solutions. A range of technologies are available to filter and destroy microorganisms inside air conditioning ducts, coils and filters and it is fairly easy to add them to most existing systems.

Ultra-violet lamps kill microorganisms, including viruses. They are often convenient to fit into the housings that contain the filters. Although there is a concern they might generate ozone, they have been shown to be very effective.

An ionized hydro-peroxide mist can be used. The great advantage of this is that it can easily penetrate around the cooling coil’s fins. It is also easy to direct it through the filters and it is very effective in destroying mould and bacterial growth.

It is often possible to retrofit older systems with coils and drip pans that have antimicrobial coatings. Copper and silver plating are both effective, and of course entirely safe to humans. Traditional folklore recommended both these metals as defences against disease long before bacteria were even discovered. The amount of silver required is very small, so nobody will be making off with your drip pans.