An attempt has been made to assess the value of counts of bacteria in the air of occupied rooms as indices of the risks of air-borne cross-infection in these rooms. Data collected during a field trial of ultra-violet irradiation in schoolrooms have been analysed to establish the correlation between counts of Streptococcus salivarius or of the general flora and classroom secondary attack rates for measles and the incidence of four groups of respiratory affections. Variations in classroom environment and child age and behaviour were taken into account in the analysis.

The Str. salivarius count has been shown to be significantly related to the risk of classroom transfer of measles, but not to the incidence of respiratory disease. For the general flora counts the reverse was true.

A discussion of these results in the light of the effects of ultra-violet air disinfection on the incidence of presumably air-borne diseases leads to the conclusion that the Str. salivarius count may reasonably be taken as index of the risk of classroom transfer of measles, but that no such assumption is possible about the value of general flora counts in the measurement of air hygiene in relation to respiratory diseases.