Many of us would probably say that poorly maintained homes and the rental market are probably the main properties that are likely to suffer from damp problems, indeed the 2009 report from the World Health Organisation on damp in the home had this to say:
Dampness and mould may be particularly prevalent in poorly maintainedHowever we know that only 36% of all homes in the UK are rented (ONS figures 2013) so those properties mentioned in the survey cannot be all rented. There must be many of the 15 million owner occupied homes that suffer from mould - and thus are damp. Many of these are likely to be well maintained so why are they damp?
housing for low-income people. Remediation of the conditions that lead to adverse
exposure should be given priority to prevent an additional contribution
to poor health in populations who are already living with an increased burden
It has been estimated that the majority of damp problems are caused by excess moisture in the air of our homes condensing on cold walls or forming 'dew' at night when the air temperature falls, especially in the cold winter months. If this moisture falls onto material that can support mould growth then all it takes is for one airborne spore to land on the surface (and there are usually at least 50-100 spores in every cubic metre of air in an average home) for mould to take root.
The problem is likely to be poor ventilation. Even the most modern home in the UK is well insulated but not likely to have enough ventilation of either passive or active kind - the focus in the UK has been on energy conservation with many government grants available for boosting insulation in the home. There is little incentive or information to ventilate, even though there are efficient mechanical devices that will remove moist air AND save the heat energy in that air, warming the incoming air.
Damp is linked to a variety of repiratory illness including asthma, allergy, reviewed here on the WHO report and several since.