How long do you have to boil dirty water for to be safe?

If you are ever in a situation, for example camping where you have to boil water from a pond or lake or if there is a big storm and the water pressure goes out and the city recommends boiling tap water, how long does it have to be boiled for it to be safe to drink? I have read different things, everything from 10 minutes, to two minutes, or that it is safe to drink as soon as it comes to a boil. What is the correct answer?

Depends on the altitude, the higher up the longer it needs boiling. This is why you see a lot of different answers!

1 min unless at high altitude. I was taught to filter through a cloth first. It could be a shirt, towel etc.
Then boil. Clearer water is easier to drink. You need to avoid puking. Hydration is important for survival.

He’s another site that lays it out

So yeah, roughly one minute full roiling boil at sea level, three minutes at 5000ft or so. Honestly though, in the situations you mention, boiling is probably a sub-par option - after all, fuel is often at a premium when camping (mass-wise) or needed for other purposes (city-wide disaster). In my 72 hour kit I keep chlorine and iodine tablets (going to taste like crap, but . . . ), and at home I have the kit plus some plain bleach for the same reasons. I would strongly advise that if you’re using such an option for camping to make sure to bring a bunch of sugar free (for mass/volume considerations) lemonade/tea/coffee powder to cover up the taste.

ETA - another reason to be careful with boiling as an option is if you’re in a Texas super freeze, you may be running risks of insufficient ventilation - as some people running generators or their cars in enclosed areas found out. Although a one burner stove is at least less likely to be an issue, it’s still one to avoid if possible. Plus, boiled water is only going to be ‘safe’ for 24 hours or so before it can be contaminated by your bottle/air/etc, the chlorine/iodine/bleach is going to be effective longer.

I typically follow pasteurization guidelines. You have to bring the whole volume to that temperature but if you’re reaching boiling you don’t have to keep it there for very long to kill everything. 0.1 seconds is plenty so if you are in a camping situation you just need to hit a rolling boil and by the time it gets below 160F you’ll have killed everything.

I carry a lifestraw. It removes a long list of pathogens and parasites.

This is the original. They offer other models.
https://www.amazon.com/LifeStraw-Personal-Camping-Emergency-Preparedness/dp/B07C56LR6N/ref=mp_s_a_1_3?dchild=1&keywords=lifestraw&qid=1632356132&sr=8-3

I ended up camping a couple weeks back at a campsite which was supposed to be front-country with water. But unbeknownst to be, the water pipe had broken, and the closest potable water was 30 miles back over really crappy roads. We weren’t prepared for that (I own a filter, but it was at home), but we were able to take water out of the river. It was pretty clear already (not glacier melt!), so all I did was boil it. I had 15 minutes in my head, so I kept it boiling way longer than I needed to. For making tea and oatmeal, it was totally fine. For drinking, it was not particularly pleasant tasting, but not overly foul either. No one has gotten giardia yet.

Should this happen again, I think I’m going to use the hit-boiling-for-a-second-and-you’re-fine rule-of-thumb.

One minute at a rolling boil according to the CDC and EPA… (3 if above 6500 feet).

Making Water Safe in an Emergency | Water, Sanitation, & Hygiene-related Emergencies & and Outbreaks | Healthy Water | CDC

The guideline I use is ‘Big bubbles, no troubles’, so if it’s a rolling boil I accept it as ‘safe’ as far as pathogens go.

If you’re talking about a boil water advisory from your municipality, then presumably boiling will take care of the problem. But if you’re taking water from a source you don’t know much about, things can get more complicated. Depending on what’s in the water, boiling may not help.

Water can be contaminated in several ways. It can contain microorganisms like bacteria and parasites that get in the water from human or animal fecal matter. It can contain chemicals from industrial waste or from spraying crops. Nitrates used in fertilizers can enter the water with runoff from the land. Various minerals such as lead or mercury can enter the water supply, sometimes from natural deposits underground, or more often from improper disposal of pollutants. Lead can leach into drinking water through old lead pipes.

Does boiling contaminated water make it safe to drink? It depends on the contaminant. Boiling water can kill germs, but things like lead, nitrates, and pesticides aren’t affected. And since boiling reduces the volume of water, it increases the concentration of those contaminants.