Published: Feb 1948 | J Exp Med. 1948 Feb 1;87(2):87-101

The lethal effects of relative humidity on bacteria

Dunklin EW, Puck TT

Abstract

The viability of three species of bacteria (pneumococcus type I, staphylococcus albus and streptococcus haemolyticus) nebulized from a suspension, was measured in a climatic chamber with adjustable different relative humidity levels and temperature.

Relative humidity had a striking effect on the decay rate! Decay rates peaked in the vicinity of 50% for all three bacteria, most significant for pneumococci (see graph).

The decay rate was strongly influenced by the properties of the suspension. Bacteria suspended in broth, 0.5 percent saline solution or human saliva showed a sharp peak in death rate around 50% RH, while bacteria suspension of sodium-free water didn’t show this pattern.

The authors made additional observations and drew some potentially crucial conclusions in view of infection prevention, which haven’t been scientifically proven over the past 50 years:

• relative humidity in the midrange of 40 to 50%, might be protective against microbial infections
• by adding vapour to an airborne suspension of dried out pneumococci, the bacteria became inactivated after having survived the desiccation procedure.

• authors hypothesized that increasing concentration of solutes, especially sodium chloride, by evaporative water loss is causal for the decay of bacteria. In intermediate humidity between 40 and 60%, quote: “moisture content reaches a critical degree at which a bacterial cell becomes much more susceptible to toxic agents than when it contains either more or less water”.