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Is it safe to use rainwater as an ingredient versus tap water - or even cooking things in it like pasta? Would there be any differences in measurements or cooking times - the thought being that there are no chemicals in it versus Tap Water?

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I'm told it goes best with pure grain alcohol. For the safety of your precious bodily fluids. – dmckee Aug 26 '10 at 18:01
+1, Good question, very original. There may not be a definitive yes or no answer, but I've wondered the same thing. – Neil Fein Aug 26 '10 at 18:15
up vote 2 down vote accepted

Keep in mind that tap water is tested to ensure it's safe for human consumption. Rainwater has no such requirement.

While rainwater seems to be a happy-fuzzy natural resource, I'd like to stress that this could be a serious safety issue. Yes, it seems unlikely, but you need an authority who knows their water.

I suggest seeking out a professional opinion on how to properly filter rainwater. Possibly from the people who handle your municipal water supply.

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The answer is obvious. We need a government sponsored regulating body to enforce rainwater purity standards! Call your congressman. – Sobachatina Aug 26 '10 at 18:48
Actually -- tap water is tested to make sure it's safe when it leaves the treatment plant. Lead solder in pipes might make it not safe by the time it reaches you, and there have been incidents in areas where additives from the treatment plant caused it to leach more lead than expected. To make things more fun, the recommended testing method for lead is to let the water run for a minute before testing -- which I don't know anyone who actually does when getting water from the tap. – Joe Aug 26 '10 at 20:25
up vote 11 down vote

You really need to filter it first. If you wouldn't drink it, don't cook with it (also good advice re: wine). There seem to be sources on the internet regarding how to make rainwater drinkable, so I would start there. (example:

If you're already drinking your rainwater, and haven't died yet, it's probably safe to cook with.

If you're planning on using it as an ingredient in baking, or any dish where the pH is critical, you may want to actually test the pH of the water you're putting in and try to balance the recipe accordingly (substitute some baking soda for baking powder, for example).

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I thought the filters on rain barrels were just to get out the particles that washed off your collection area like leaves and gravel. I don't think it is to remove impurities from the rain. – Sobachatina Aug 26 '10 at 16:54
Anything that could be removed by a charcoal filter will have already been removed when the water evaporated and subsequently condensed and fell from the sky. – hobodave Aug 26 '10 at 16:56
@hobodave Rain can pick up particles on it's way down, though. At any rate, I don't think anyone here is really an expert on drinking water safety, so I stand by my answer that you should only cook with water that's already known to be safe to drink. I think it's irresponsible to suggest otherwise. – Bob Aug 26 '10 at 17:01
I'll second filtering. Rain requires something to condense on in order to form, so there MUST be at least some dust involved. Having been in rainstorms that left my windshield to muddy to see through... I wouldn't trust rain to be clean. It will be free of artificial chemicals, such fluoridation, however. – Scivitri Aug 26 '10 at 17:16
@Sobachatina : the filters on rain water barrels are also to keep them from turning into mosquito breeding tanks. – Joe Aug 26 '10 at 20:19

As long as you don't live in an area with bad acid rain, and you are catching it in a sanitary container without running it through gross gutters etc, it would be fine.

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Rainwater, straight out of the sky, is generally safe, with some caveats depending on the location. For example, in a polluted area, it's probably not a good idea.

The 2 biggest issues are primarily how it's collected and how it's stored. Collection and storage are where rainwater can easily become contaminated by either chemical pollutants or biological pathogens. So, if you want to replace your tap water with rainwater, it'll take an investment in sanitary collection and storage devices/methods.

BTW. On desert hiking trips I have used rainwater collected from a tarp dripping into a pan and then filtered through a bandanna to remove dust. I had no worries since the terrestrial sources were questionable.

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+1, this is common sense. You could say that all water is rainwater -- just some of it was rain more recently. – slim Mar 9 '11 at 14:38

I would happily drink rain water in any amount, in most situations. But how that rainwater is collected and stored is a big deal. For example:

  • Roofs often have dirt and animal feces on them, and many roofing materials are treated with toxic chemicals.

  • Dirty, standing water is a breeding ground for bacteria, algae, and insects.

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As long as there hasn't been a recent chemical, biological, or nuclear attack you should be fine. Acid rain shouldn't be much of a problem since it's only slightly more acidic than regular rainfall (which is already acidic).

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That's a trippy answer :>) – AttilaNYC Aug 26 '10 at 17:01

It is safe to use water from streams, rivers, and lakes in the US (not including those affected by nuclear power plants) if you boil the water to a rolling boil and keep it there for 3 minutes first. This kills bacteria which commonly live in bodies of water.

Rain water is unlikely to have giardia living in it, unlike streams, but sources indicate that it may contain some bacteria depending on the collection method. Therefore keeping your rain water at a rolling boil at a minimum for safety purposes.

Note that at least one scientist from Argonne National Labs also recommends filtering, but several others on the same site indicate that rain water should be perfectly safe.

Does this question mean we will shortly see rainwater cooked meals infiltrating the trendiest restaurants?

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