Is it because of the water sitting in the reservoir for so long?
http://en.wikipedia.org/wiki/Windshield_washer_fluid
Varieties
Windshield washer fluid is sold in many formulations, and some may require dilution before being applied, although most solutions available in North America come premixed with no diluting required. The most common washer fluid solutions are given labels such as “All-Season”, “Bug Remover”, or “De-icer”, and usually are a combination of solvents with a detergent. Dilution factors will vary depending on season, for example in winter the dilution factor may be 1:1, whereas during summer the dilution factor may be 1:10. It is sometimes sold as sachet of crystals, which is also diluted with water. Distilled water is the preferred diluent, since it will not leave trace mineral deposits on the glass.
Anti-freeze, or methylated spirits, may be added to a mixture to give the product a lower freezing temperature. But methanol vapor is harmful when breathed in, so more popular now is an ethanol winter mix, e.g. PAV[clarification needed], water, ethanol (or isopropanol), and ethylene glycol.
Concerns have been raised about the overall environmental aspects of washer fluid.[citation needed] Widespread, ground-level use of wiper fluid (amounting to billions of liters each year)[citation needed] can lead to cumulative air pollution and water pollution.[citation needed]
Consumer advocacy groups and auto enthusiasts believe that the alcohols and solvents present in some, but not all, windshield washer fluid can damage the vehicle.[citation needed] These critics point to the corrosive effects of ethanol, methanol, and other components on paint, rubber, car wax, and plastics, and groups propose various alternatives and homemade recipes[specify] so as to protect the finish and mechanics of the motor vehicle.
Legionnaires’ disease
On 14 June 2010, the UK’s Health Protection Agency announced the results of a preliminary study of 75 patients, which found an association between the use of plain water as wiper fluid and Legionnaires’ disease, which is spread by breathing in aerosolized bacteria from infected water. It had been noticed that prevalence of the disease was five times higher among professional drivers.[2][3]
Good grief. Never seen so many “citation needed” entries. Looks like some jackass decided that was the next-big-thing to get bent out of shape over.
One would think that the most likely source for Legionnaire’s disease for passengers would be the liquid condensation that can accumulate in the AC system.