Clear water is not safe water
A cold, clear mountain stream looks like the cleanest water there is. The CDC's page on water treatment for hiking, camping and travel says otherwise: water that looks clean can carry bacteria, viruses and parasites from human or animal waste, and drinking straight from a clear stream is generally not safe. The Great Smoky Mountains National Park hiking page gives a local example, warning that stream water there may contain the parasite Giardia, which can cause intestinal illness weeks after you drink it.
The CDC also reminds readers that untreated water causes problems beyond drinking: using it to cook, wash food, make ice or brush your teeth can make you sick too. The common symptoms it lists are diarrhoea, vomiting and stomach pain. Children, pregnant women, people aged 65 or older and people with weakened immune systems are at higher risk.
This is why the National Park Service's ten essentials list pairs water with water treatment supplies, and asks visitors to find out before the trip whether there is water along the route that they could collect and treat.
How to treat water
The CDC sets out a clear order of preference. Boiling is the best way to kill germs. If you cannot boil, the next best option is to filter the water and then disinfect the filtered water. Whatever product you use, follow its instructions.
Boiling
The CDC says boiling kills all types of germs, including viruses, bacteria and parasites. Bring clear water to a rolling boil for 1 minute. At elevations above 6,500 feet, boil it for 3 minutes. Let it cool before you use it, to avoid burns.
Filtering
Filters differ a great deal, and the CDC explains what the labels mean. Most portable filters remove parasites but not viruses or bacteria. A filter with an absolute pore size of 1 micron or smaller, or one certified to NSF standard 53 or 58 for cyst reduction, removes parasites but not viruses or all bacteria. A pore size of 0.3 micron or smaller removes bacteria and parasites, but still not viruses. Reverse osmosis filters remove all three, and salt as well.
Disinfecting
Chemical disinfectants such as unscented household chlorine bleach, iodine or chlorine dioxide tablets kill bacteria and viruses, but the CDC notes that chlorine and iodine do not work well against parasites. Chlorine dioxide can kill Giardia and is somewhat effective against Cryptosporidium, but filtering first is still the safest approach. The CDC advises people who are pregnant, have thyroid problems or are sensitive to iodine to avoid iodine-treated water, and says nobody should drink it for more than a few weeks at a time.
Portable ultraviolet light units can kill parasites, bacteria and viruses in small amounts of clear water, but the CDC says ultraviolet light does not work well in cloudy water, because particles shield germs from the light. Filter first.
What treatment cannot fix
The CDC is clear that boiling and disinfecting cannot make water safe if it contains harmful chemicals, toxins or radioactive material. If you suspect that kind of contamination, use a different source.
Methods compared
| Method | Parasites | Bacteria | Viruses |
|---|---|---|---|
| Rolling boil (1 minute; 3 minutes above 6,500 feet) | Yes | Yes | Yes |
| Filter, 1 micron or NSF 53/58 cyst reduction | Yes | Not all | No |
| Filter, 0.3 micron or smaller | Yes | Yes | No |
| Reverse osmosis filter | Yes | Yes | Yes |
| Chlorine bleach or iodine | Not well | Yes | Yes |
| Chlorine dioxide | Giardia yes; Cryptosporidium partly | Yes | Yes |
| Ultraviolet light (clear water only) | Yes | Yes | Yes |
Treating water is only part of keeping it clean. The CDC also advises burying human waste well away from water where there are no toilets, and washing hands before handling food and after using the toilet, using a hand sanitiser with at least 60% alcohol when soap and water are not available. The public lands guide covers waste disposal in more detail.
How much to carry
There is no single national figure for how much water a hiker needs, because it depends on heat, effort, terrain and the person. The Park Service's ten essentials page gives the principle: physical activity raises the risk of dehydration, so when you are active outdoors, especially in hot weather, drink often and before you feel thirsty, and prepare your water before you need it.
Individual parks publish figures for their own conditions. Great Smoky Mountains, for example, suggests at least one liter for every two hours of hiking, and about 3 to 4 quarts a day to help prevent heat illness. Other parks publish their own figures for their own climates, so read the page for the park you are visiting. The CDC's heat page offers a simple check: light yellow or clear urine usually means you are drinking enough.
Who heat affects most
Heat can make anyone ill. The National Weather Service's heat page notes that it can also make existing health conditions worse, and names groups that are particularly vulnerable: infants and young children, older adults, people with chronic medical conditions and pregnant women. The CDC's heat page adds people with asthma or heart disease, and, directly relevant here, people who are working or exercising outside in the heat.
The CDC also notes that many medicines can make people more likely to become dehydrated or overheated, and says not to stop or change a medicine without first talking to your doctor. If you take regular medication and plan a strenuous trip in hot weather, that conversation belongs in the planning.
The CDC's practical advice for hot days outdoors is to stay in the shade as much as possible, take breaks, plan activity for the coolest parts of the day, and carry a water bottle and refill it. Its HeatRisk forecast is one way to check how dangerous the heat is likely to be where you are going.
Recognising heat illness
Heat illness ranges from cramps to a medical emergency, and the earlier it is spotted the easier it is to stop. The CDC lists general signs that the body is overheating: muscle cramps, unusually heavy sweating, shortness of breath, dizziness, headache, weakness and nausea.
The CDC's National Institute for Occupational Safety and Health describes the main conditions in more detail. Its page is written for workers, but the signs are the same for anyone exerting themselves in heat:
- Heat cramps: painful muscle cramps or spasms after heavy sweating during hard activity.
- Heat exhaustion: the body's response to losing too much water and salt, usually through sweating. Signs include headache, nausea, dizziness, weakness, irritability, thirst and heavy sweating.
- Heat stroke: the most serious. Signs include confusion, slurred speech, loss of consciousness, hot dry skin or heavy sweating, and seizures. It can cause permanent disability or death without emergency treatment.
Heat stroke is an emergency
Call 911. The NIOSH page says to stay with the person, move them to a shaded, cool area, remove outer clothing and cool them quickly, for example with cold water, wet cloths or by soaking their clothing. The Great Smoky Mountains page gives the same instruction to call 911 immediately.
For heat exhaustion, the NIOSH page advises getting the person out of the heat, giving them liquids, removing unnecessary clothing, cooling them with cold compresses or cold water on the head, face and neck, encouraging frequent sips of cool water, and getting medical evaluation. On a trail, that usually means stopping in the shade and ending the hike.
Sun on skin and eyes
Sun protection is one of the ten essentials. The CDC's sun safety page explains why it matters outdoors in every season: ultraviolet rays reach you on cloudy and cool days, and reflect off water, sand and snow. In the continental United States, they are strongest from 10 a.m. to 4 p.m. daylight saving time, and the CDC advises protecting your skin whenever the UV index is 3 or higher.
Its practical steps are shade; long sleeves and long trousers in tightly woven fabric; a hat with a brim all the way round; sunglasses that block both UVA and UVB rays; and a broad-spectrum sunscreen with SPF 15 or higher, reapplied if you stay out more than 2 hours and after swimming, sweating or towelling off. A wet T-shirt protects less than a dry one, which is worth remembering on a paddling trip.
Example: a hot-day plan
Worked example
This is an illustration and describes no real people or place. A family plans a summer day hike on an exposed trail with one stream along the way.
- They check the park's website for its water advice and the heat forecast, and choose to start early.
- Each person carries filled bottles, and the group carries a filter and a disinfectant to treat stream water for the return.
- Hats, long sleeves, sunglasses and sunscreen go on at the trailhead, with sunscreen reapplied after two hours.
- They take shaded breaks and drink before they feel thirsty.
- When one person says they feel dizzy and sick, the group stops in the shade, cools them with wet cloths and water, and turns back. They agree that any sign of confusion would mean calling 911 straight away.