Key takeaways
Microbial contamination is the biggest water-related health risk worldwide, while in most high-income cities regulated tap water is safe to drink
Lead from old pipes, nitrates in rural wells and PFAS near contamination sites are the main chemical concerns to check locally
Water hardness is not a health concern according to WHO, and for training what matters most is drinking enough, not which water you drink
Water is the thing you consume most of, so it's natural to worry about it. Social media adds plenty of noise: alkaline water, "structured" water, and filters marketed as essential for everyone. The real picture is more reassuring for most people in countries with a regulated supply, with a few specific risks worth checking.
The global picture
According to WHO, 2.2 billion people lacked safely managed drinking water in 2022, and at least 1.7 billion used a source contaminated with faeces. Microbes, not chemicals, cause most of the harm: diarrhoeal disease from unsafe water, sanitation and hygiene kills around a million people a year.
If you live in a city with a regulated public supply, your water is disinfected and tested against national standards. Problems still happen, but they tend to be local and specific rather than universal.
The contaminants that actually matter
| Contaminant | Main source | Who is most at risk | Reference value |
|---|---|---|---|
| Microbes (E. coli, norovirus, Giardia) | Sewage, flooding, untreated wells | Everyone, especially children | Should not be detectable |
| Lead | Old service lines, solder, brass fittings | Infants, children, pregnancy | WHO provisional 10 µg/L; EPA goal zero |
| Nitrate | Fertiliser and manure in rural groundwater | Bottle-fed infants | WHO 50 mg/L |
| PFAS ("forever chemicals") | Industry, firefighting foam | Communities near contamination | EPA 4 ng/L for PFOA and PFOS |
| Hardness (calcium, magnesium) | Natural geology | Nobody, it is not a health risk | No health-based limit |
Lead
US EPA states that there is no known safe level of lead in a child's blood and sets its health goal for lead in water at zero. Lead mostly comes from plumbing, not from the treatment plant: lead service lines, older solder and brass taps, especially in homes built before 1986 in the US. WHO keeps a provisional guideline of 10 µg/L but says levels should be as low as reasonably practical. Boiling does not remove lead.
Nitrate
WHO's guideline of 50 mg/L protects bottle-fed babies from methaemoglobinaemia. It is mainly a concern for private wells in farming areas. Don't confuse it with the dietary nitrate in beetroot juice that some athletes use, which is covered in our supplements guide. Infant safety, not sport, sets the water limit.
PFAS
In April 2024 the EPA set the first national limits for six PFAS, including 4 parts per trillion each for PFOA and PFOS. In 2026 the EPA proposed giving utilities until 2031 to comply with those two limits and rescinding the limits for the other four, so check the current status. Whatever the rules, your utility's PFAS results are often public.
Hardness
Hard water causes limescale and changes taste. WHO's background document concludes that some studies suggest a protective link between magnesium or hardness and cardiovascular deaths, but the evidence "does not prove causality", so it proposes no guideline. Don't buy a softener or hard-water minerals for your heart. Exercise and blood pressure control do far more, as covered in VO2max and heart disease.
Microplastics
A 2019 WHO review found no evidence of a health risk from microplastics in drinking water at current levels, while stressing that data were limited. Bottled water is not a way to avoid them.
Tap, bottled or filtered?
- Tap water from a regulated supply is the default sensible choice: cheap, tested, and usually low in sodium.
- Bottled water is useful when the local supply is unsafe or during boil-water notices. Elsewhere it mostly adds cost and plastic.
- Filtered water helps when a filter targets a known problem. EPA advises checking for certification to NSF/ANSI 53 or 58 for PFAS reduction. A jug filter improves taste and chlorine smell but may not remove lead or PFAS unless certified to do so. Heavier options like reverse osmosis remove more, including minerals.
How to check your water
- Read the annual report. US utilities publish a Consumer Confidence Report every year, and most national regulators publish similar data.
- Ask about your pipes. Utilities increasingly publish lead service line maps.
- Test private wells yourself. CDC recommends testing at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH, and more often after flooding, during pregnancy or when a baby joins the household.
- Use an accredited lab for lead or PFAS rather than cheap strips.
- While you wait, use cold water for drinking and cooking, and run the tap after the water has sat in the pipes.
Water isn't the only home exposure people ask about. Damp walls are more common, especially in humid climates; see mold exposure.
Water, hydration and performance
No study shows that one safe water source improves VO2max more than another. What matters for performance is drinking enough. The ACSM position stand on fluid replacement advises avoiding a water deficit above about 2% of body weight during exercise, because larger losses impair performance, especially in the heat. In long sweaty sessions, drinks containing electrolytes and carbohydrate can help more than water alone.
See heat training for hot-weather sessions, use HRV and resting heart rate to spot poor recovery, remember that training moves the needle most (how to improve VO2max), and track your fitness with the VO2max calculator. Daily habits like morning daylight probably matter more for how you feel than your choice of filter, and marketed extras like red light therapy deserve the same scepticism as "miracle" waters.
Frequently asked questions
Is tap water safer than bottled water?
In places with a regulated public supply, tap water is tested more often than many people realise and is usually as safe as bottled water. Bottled water makes sense where the supply is unsafe, during boil-water notices, or when travelling.
Is hard water bad for your health?
No. WHO does not set a health-based limit for hardness because it is not a health concern at levels found in drinking water. It mainly causes limescale and affects taste.
How do I know what is in my tap water?
Read your water supplier's annual quality report, check national or state databases, and test your own tap if you have old plumbing, a private well or a specific concern.
Do I need a water filter?
Only if a known problem exists, such as lead pipes, PFAS above local limits or a well with nitrates. Choose a filter independently certified for that specific contaminant and change cartridges on schedule.
Sources
- World Health Organization. Guidelines for drinking-water quality: fourth edition incorporating the first and second addenda. Geneva, 2022.
- World Health Organization. Drinking-water fact sheet. who.int, accessed 2026.
- World Health Organization. Hardness in drinking-water. Background document for the Guidelines for Drinking-water Quality, WHO/HSE/WSH/10.01/10/Rev/1, 2011.
- World Health Organization. Microplastics in drinking-water. Geneva, 2019.
- US EPA. PFAS National Primary Drinking Water Regulation (April 2024) and proposed 2026 changes. epa.gov/sdwa.
- US EPA. Reducing PFAS in your drinking water with a home filter. Fact sheet, 2024.
- US EPA. Basic information about lead in drinking water. epa.gov.
- CDC. Guidelines for testing well water. cdc.gov.
- American College of Sports Medicine, Sawka MN, et al. Exercise and fluid replacement. Position stand. Med Sci Sports Exerc, 2007.