Light and moderate raindrop impacts can release soil bacteria into the atmosphere through a microscopic bubble bursting process, with the effect declining at higher impact speeds, according to laboratory research published in Nature Communications.
The study conducted using individual raindrops, sterilised soils and cultured bacteria, found that when a raindrop strikes porous, partly dry soil, it can trap small pockets of air beneath the spreading water. These bubbles rise through the drop and burst at the surface, producing tiny droplets that can carry bacteria and other material from the soil into the air.
Researchers found that bacterial aerosol production generally increased with drop speed before reaching a soil-dependent peak at around 1.4–1.7 metres per second. At higher speeds, production through this bubble-mediated mechanism declined as the raindrop spread more violently and formed a thinner liquid layer.
The findings do not mean that every light rainstorm releases more bacteria than a heavy downpour. The experiments measured individual drops under controlled laboratory conditions, while natural rainfall involves different drop sizes, speeds, rainfall intensities and changing soil conditions.
Soil type plays a major role
The researchers tested six types of soil, including clay, sandy clay and sand. Sandy clay produced the highest levels of bioaerosols in the experiments.
Clean sand, by contrast, absorbed the drops rapidly, limiting the formation of the trapped air pockets required for the bubble bursting mechanism. No bioaerosol was detected from the sandy samples under the tested conditions.
The results suggest that the same rainfall may produce very different aerosol emissions depending on soil texture, pore structure, moisture and how quickly water penetrates the ground.
Surface temperature also influenced particle production, with the study finding substantial differences between favourable and unfavourable temperatures in some soil types.
Bacteria remained viable after aerosolisation
To test whether living microorganisms could survive the process, researchers introduced laboratory strains of Corynebacterium glutamicum Bacillus subtilis and Pseudomonas syringae into sterilised soils.
The bacteria were detected in droplets collected above the soil after raindrop impacts, demonstrating that the bubble bursting process can transfer viable cells into airborne droplets.
However, the experiment did not establish whether the released bacteria could cause infections or how long they would survive in the atmosphere. Factors such as sunlight, humidity, wind and atmospheric exposure were not examined.
Some individual drops generated more than 100 aerosol particles smaller than 10 micrometres, while imaging also detected much smaller droplets. Such particles can potentially remain airborne longer than larger splash droplets, although the study did not track their movement under natural conditions.
The researchers estimated that a favourable raindrop could transfer roughly 0.01 per cent of bacteria present on the relevant soil surface. They also produced a global estimate ranging from 1.2 × 10²² to 8.5 × 10²³ bacterial cells potentially aerosolised by raindrop impacts each year.
That global figure is an extrapolation rather than a direct measurement. It depends on assumptions about bacterial concentrations, soil coverage, moisture and the proportion of rainfall occurring under conditions favourable to bubble formation. The authors also regarded the estimate as an upper bound because subsequent rainfall can remove airborne particles through washout.
The study therefore provides evidence for a specific physical pathway by which rain can move microorganisms from soil into the atmosphere. It does not establish that light rain universally produces more airborne bacteria than heavy rain, nor does it demonstrate a resulting increase in human disease or other health effects.
Further field research would need to examine natural bacterial communities during real rainfall while accounting for rainfall intensity, raindrop size, soil texture, moisture, temperature, wind and vegetation. Such measurements would help determine how important the laboratory mechanism is during actual storms.