Modern dental units may contain sophisticated electronics, but in addition they are dependent upon air lines (conveying pressurized air) and also water lines, known as Dental Unit Water Lines (DUWLs). DUWLs are almost designed to encourage Biofilm development, as they are long and thin, have a laminar water flow and are almost impossible to dry out completely.
It is not surprising to find that water sampled from DUWLs typically yields fungi and protozoa. In addition soil and aquatic environmental bacteria, typically Gram-negative, are in abundance. Opportunist pathogens such as Legionella spp, Pseudomonas aeruginosa and Non-Tuberculous Mycobacterium spp are also often found. Given the abundance of Gram-negative bacteria it is not surprising that high levels of endotoxins are also often found. Legionella spp were isolated from 68% of DUWLs, with L. pneumophilia present in 8%. Pseudomonas aeruginosa was isolated from 24% of DUWLs and despite cleansing was repeatedly isolated from individual units. Non-Tuberculous Mycobacterium spp are found at a mean level of 365 Colony Forming Units/ml. Endotoxins in DUWLs reached levels of 2,560 EU/ml compared to 66 EU/ml in tap water or <0.25 EU/ml in water for injection.
Detailed epidemiological studies are rare. There are some proven links directly between disease, even death, and infections derived from DUWLs – these are detailed under The Issues section. However there are some general conclusions.
Dentists and their staff have higher rates of respiratory infections than the general public. Dental staff have higher rates of sero-positivity for Legionella antibodies than the general public, titres appear related to the time they spend in clinics. Occupational asthma rates in dental staff increase with their exposure to practice. The temporal onset of asthma may be associated with occupational exposure to contaminated DUWL among dentists.