Humidifier fever.

UMIDIFIER FEVER is classed as an example of extrinisic allergic alveolitis (EAA)’ (or hypersensitivity pneumonitis)l on the basis of time of onset of symptoms after exposure, clinical manifestations and presence of antibodies against the offending agent. In humidifier fever the hypersensitivity reaction is to microbial antigens developing usually in humidifiers and disseminated via air conditioning systems. Inhalation and interaction between these antigens and the immune system of the susceptible individual produces an episode of humidifier fever some hours after inhalation3. The episode is similar to a bout of ’flu’ except that the patient is usually better the next day when no further problems may occur even on exposure to the same environment. This refractory period is seen in other diseases in the EAA group4 and persists in humidifier fever until a significant break from the inducing environment presumably allows build-up of the pyrexia inducing factors. The next attack may then occur, for example, after a weekend break but may not necessarily be so debilitating as to deter people from continuing their employment even with regular Monday attacks. It is perhaps fortunate that this example of EAA has not been seen to progress to the chronic stage that is seen in other examples of EAA, eg farmer’s lung, pigeon breeder’s lung’, perhaps the nature of the offending antigen is responsible for I this.