All-in-all, a properly installed, well-maintained air conditioning system is unlikely to cause health problems in most people.

That said, it's better not to use one, or to use one as little as possible.

In the broader environmental scheme of things, air conditioning uses energy, and contributes to pollution and global warming. However, as regards immediate human health issues, the argument against aircon is somewhat harder to make.

Many studies over the past two decades, and several reviews of research over this period, have found that occupants of office buildings with air-con report, on average, more symptoms than workers in buildings with natural ventilation. The symptoms tend to be quite a mixed bag, from headaches and dizziness to various respiratory tract irritations, and are seldom linked to a specific cause. Probably the most common and likely contaminant in aircon systems are mold spores, from mold growing in damp, poorly maintained systems. Some people, often those who already have asthma or allergy, are much more susceptible to 'sick building syndrome' than others.

It is possible for infectious viral and bacterial diseases to be spread by air conditioning units, but such cases are quite rare – or at least not well documented.

To give it its due, air conditioning significantly decreases the risk for heat-related illness (e.g. heat stroke), particularly among vulnerable groups like the elderly and children.

If you notice symptoms on waking e.g. dry mucous membranes, itchy throat, then give the air con a break for a while.