Water dripping from air conditioners: Is it sanitary?

Answers to your questions about the news.
July 15 2013 3:02 PM

How Gross Is the Water That Drips From Air Conditioners?

It's like rain on a summer evening.

Celebrate Air Conditioning Appreciation Week with this gallery  from Magnum Photos. 

Ever wonder about the water that drips down from air conditioners as an unwelcome sidewalk surprise when you’re walking to work? In 2011, Forrest Wickman looked into the reason whybACs are so leaky, and if there’s anything unsanitary about the water that’s dripping down on you. The original article is reprinted below.

Air conditioner. Click to expand image.
Air conditioner water: Is it good enough to drink?
Forrest Wickman Forrest Wickman

Forrest Wickman is a Slate staff writer. 

Walk down any city sidewalk on a hot summer day, and you're bound to get wet—and not just when it's raining. Water drips from window AC units, especially on muggy days, and this unpleasant drizzle can fall into your hair or even onto the lip of your morning coffee cup. Is all that dripping water sanitary?

Yes, as a general rule. Most of the dripping from air conditioners is just condensed water vapor that comes from the air inside the building. Window air conditioners are designed to drain this water from the back, raining it down on any unsuspecting pedestrians below. In most ways this water is exactly like rain (which also forms from condensed water vapor) or the moisture that collects on a cool can of soda, and it's typically no more harmful. However, in rare cases small amounts of water can be left to stagnate inside the air conditioner, making it a breeding ground for bacteria.

Click to launch slideshow.

On a hot and humid day, a window unit can drip up to 2 gallons of water, which accumulates on its evaporator coil as it cools and dehumidifies the air. (Very little condensation gathers on the exterior side of an AC, which tends to be warmer than the air around it.) This coil, like many plumbing pipes used for drinking water, is made of copper (which is also what makes air conditioners so heavy), and it's much cleaner than you might expect from looking at a dusty AC filter. While copper can be unhealthy in high doses, the condensate from air conditioners seems to be low in minerals and dissolved solids.

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In a properly functioning air conditioner, the water drips down from the coil into a condensate pan and then exits the unit through a drain or tube. However, a clog in this drain or tube can leave a puddle to accumulate inside, which is an ideal environment for many types of harmful bacteria. In particular, a 1976 outbreak of Legionnaires' disease was caused by bacteria that spread out of the air conditioning system at Philadelphia's Bellevue-Stratford Hotel. (That's how the disease got its name: Many of the victims were attending an American Legion convention.) While Legionella is known to thrive in the cooling towers of large air conditioning systems like the one at that Philadelphia hotel, it does not seem to grow in smaller units. Furthermore, dripping water isn't really stagnant, so it's extremely unlikely that the water raining down on pedestrians would be infected.

The water that drips from air conditioners is probably even safe for drinking. (It's certainly more potable than the drinking water in many countries.) Still, for the reasons mentioned above, it's best not to tilt your head back for a draft. If you're looking for a better use for your air conditioner's condensate, the Explainer recommends using it to water your plants.

Got a question about today's news? Ask the Explainer.

Explainer thanks Douglas T. Reindl of the University of Wisconsin-Madison and Mark Sobsey of the University of North Carolina at Chapel Hill.

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