What is the “infective dose” for contracting Legionnaires’ disease?

I am an industrial microbiologist currently employed as a consultant for the management and control of Legionellosis to a large number of hospitals in the UK. A question which is now being asked with increasing frequency is the matter of the “infective dose”. Is there an infective dose for Legionella?

For cooling towers, no such dose has been calculable. (One reason it has been well nigh impossible to calculate such a dose is that cooling towers are probably not the source of the legionella in most of the reported outbreaks).

For water distribution system, it has been established that quantitative counts of Legionella from the distal fixtures (faucets) do not correlate with the incidence of the disease in hospitals. On the other hand, 4 studies have found that when distal site positivity is 30% or higher, cases of hospital-acquired Legionnaires’ disease occur (Stout JE. Infect Control Hosp Epidemiol, 2007).