How Bacteria in Rain Water Can Pose Health Risks

Despite childhood myths, it’s not sterile at all

“Come out of the rain, it’s full of germs!”Photo byJamie Brown on Unsplash

Growing up, I heard that rain was clean, that it was the purest water I could drink. That’s my excuse for why I ran around outside in the rain with my mouth open, staring up into the sky, and I’m sticking with it.

But rain isn’t clean. Even the puffy white clouds in the sky aren’t clean. Clouds are full of bacteria.

Could those bacteria make us sick? Could drinking rainwater, in fact, lead to illness?

All the stuff in rain besides water

Our atmosphere is full of stuff, tiny little particles that are light enough to drift in the air currents instead of settling back down to earth. As rain falls through the air, it picks up all sorts of stuff.

This contamination begins from the very beginning. Inside clouds, raindrops form as tiny bits of water vapor collide and merge. But it’s much easier for raindrops to coalesce around an already existing particle in the atmosphere, like a tiny grain of desert dust, pollution, or a bit of organic matter.

These particles can significantly alter the appearance and qualities of the rain itself! Chemicals like sulfur dioxide and nitrogen oxides mix with the raindrops and make them more acidic, leading to acid rain.

But these are all inorganic materials. What about bacteria or other living organisms?

High-altitude, frozen bacteria

It’s hard to study clouds; they’re hard to reach and they are diffused over a very large area. Fortunately, they drop samples that we can collect.

In one study, researchers collected rainfall from North China grasslands and extracted DNA from the samples to find out which bacteria, if any, were present. Despite the rain falling at freezing temperatures, it contained a wide range of both bacteria and fungi. Across all samples, 908 different strains of bacteria and 470 strains of fungi were detected, although a few specific groups of bacteria tended to be most dominant.

Algae can also spur raindrop formation. In one extreme example, tiny single-celled algae, typically found in rainforests, were swept up into clouds. Those clouds later rained the algae down into India, where they gave the rain a terrifying “blood-like” red tinge.

One Nature paper reported that the most common bacteria were Pseudomonas syringae, Xanthomonas campestris, and Pantoea ananatis, and the most common fungi included Alternaria sp., Fusarium sp., Cladosporium sp., Phoma sp., Rhizopus sp., and Botrytis cinerea.

These bacteria and fungi are well characterized as pathogens— but not to humans.

To plants.

This makes sense, from an evolutionary perspective. If you’re a bacterium, it takes a lot of energy to maintain the systems to let you survive the freezing environment of the upper atmosphere. There has to be a payoff — such as spreading to new hosts.

And plants, in turn, have evolved to combat these paratrooper microbes. When plants are bombarded by tiny water droplets, they activate immune responses, seeking to fend off any potential pathogen invaders.

That’s great, but what about me? Can I get sick from rain?

Thankfully, there haven’t been cases of widespread human pathogen distribution through rain. Coronavirus is not falling out of the skies in raindrops.

But rain can still range from “probably fine” to “definitely bad to drink,” depending on where you live and what’s around you. There’s two main issues:

1. Any pollution in the air is mixed with the rain;
2. Most rain collection methods are not very clean.

Issue 1 means that, even if you “drink” by standing out in the rain with your mouth open, you’re going to be drinking water that may contain pollutants. Studies have shown that toxic plastic-associated chemicals can be found in rain, and rain near a large city or smoggy area can carry these in the droplets.

Issue 2 is more around collecting rainwater in cisterns or barrels for longer-term use. Rainwater collection systems also pick up dirt, lead, copper, and asbestos dust, and often accumulate bird feces as well — and that’s before touching on how stagnant water can provide a breeding ground for both bacteria, algae, and insects!

Rainwater can be purified and treated, such as by boiling to kill bacteria and filtering it to remove chemical contaminants. Once properly treated, rainwater is no more dangerous than any other water source.

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Rain isn’t clean. It’s got all sorts of microparticles, both organic and inorganic, floating inside those droplets. Most of the bacteria and fungi found in rain prey on plants, not humans or other animals, but it’s certainly not as pure as, say, reverse-osmosis filtered water. (And there’s no such thing as too-pure water, either!)

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Standing out in the rain occasionally isn’t likely to make you ill. But rainwater is likely more polluted than other water sources, and it shouldn’t be your primary source of hydration.

And next time it rains, don’t forget to tell your friends how those raindrops are full of bacteria!

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