It depends on the details.
EPA says contaminants in drinking water can affect health, including over the long term, depending on which contaminant, how much, for how long and who is drinking it. That doesn't mean every tap is unsafe. It means the details of your own water matter.
In EPA's own words.
"…can cause health effects, such as gastrointestinal illnesses, nervous system or reproductive effects, and chronic diseases such as cancer."
On the same page, EPA explains that lower doses over long periods can lead to chronic, longer-term conditions such as cancer, and that the effects of some drinking-water contaminants are not yet well understood.
EPA lists the factors that decide whether a contaminant leads to health effects:
- the type of contaminant
- its concentration in the water
- how susceptible the person is
- how much water they drink
- how long the exposure lasts
Summary of EPA, Drinking Water · page last updated May 27, 2026
What a "lifetime" health advisory is.
For some contaminants, EPA publishes health advisories: levels below which health effects are not expected over a given exposure period, for example "(e.g., 1 day, 10 days, a lifetime)."
EPA says advisories are not legally enforceable standards. They inform water systems and officials, especially during spills or contamination events, and they can change as new information becomes available.
Summary of EPA, Drinking Water Health Advisories (HAs) · page last updated Mar 30, 2026
What this does not mean.
- that your water, or any particular water system, has unsafe levels of anything
- that drinking tap water causes cancer or any other disease
- that water above an advisory level is illegal or will cause harm
What they do show is that the questions worth asking are specific: which contaminants were measured, at what levels, when, and how those levels compare with the legal limit and EPA's health goal.
On a private well? USGS notes that EPA regulates public water supplies, while homeowners are responsible for testing and treating private wells. USGS, Jul 5, 2023
- EPA · Drinking WaterPage last updated May 27, 2026 · quote + summary · Last checked Oct 9, 2026
- EPA · Drinking Water Health Advisories (HAs)Page last updated Mar 30, 2026 · quote + summary · Last checked Oct 9, 2026
- USGS · Tap water study detects PFAS ‘forever chemicals’ across the USPublished Jul 5, 2023 · summary · Last checked Oct 9, 2026
Plain-English summaries are ours. Words in quotation marks were checked word for word against the agency page on Oct 9, 2026. Links go to the agency's own site. EPA and USGS do not endorse this site or any product. This is general education, not medical advice.