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The combination of biofilm formation and the use of the contaminated water are of high concern during patient treatment in health care and the dental environment [10]. The presence of a biofilm constantly delivers a number of detached microorganisms in a planktonic stage to the output water used for treatment procedures, such as spraying, cooling, and ultrasonication, which all form aerosols. Aerosols are easily inhaled by patients as well as the dentist and nurse. The personnel can protect themselves with a mask that should cover both the mouth and nose to fulfil its purpose. However, the patients cannot be protected and may be in a more vulnerable situation since the aerosols are formed in and around their mouths and they are exposed to a higher degree to the aerosols by inhalation. It should be pointed out that the contaminated water from DUWLs is not hazardous to drink, since gastrointestinal pathogens such as coliforms are extremely uncommon in DUWLs [2–4]. However, high numbers of microorganisms should be regarded as poor quality and in the worst cases the water can even be discolored and smell and taste bad, and unacceptable for used for irrigation in the mouth. With a high number of microorganisms in DUWLs, the potential risk is entirely connected to the risk of inhalation and development of respiratory tract infections. This is especially important in immune and medically compromised patients, the elderly, and other individuals more susceptible to developing respiratory tract infections, who can even develop infection from normally innocent microorganisms or their products (e.g., endotoxins) if they are exposed to a high number of microorganisms. The risk of complications increases when respiratory pathogens are present (see below).

The risk with aerosols is well known from other similar systems, such as air conditioners, air moisteners, water towers, and showers. In fact, the first well-recognized incidence with respiratory tract infection occurred in 1976 with the outbreak of pneumonia caused by Legionella among veterans who had assembled in Philadelphia (PA, USA) and were exposed to an air condition system containing L. pneumophilia [11]. Since then, a number of similar outbreaks have been reported in hotels, old people’s home, and hospitals [11]. Although only a few case reports are documented in dentistry [12], the theoretical risk of gaining respiratory tract infection from DUWLs is considered to be high. However, the actual frequency of respiratory tract infections caused by aerosols from dental unit water is unknown due to difficulties in tracing the origin of the bacteria.