How the EPA Sets Drinking Water Standards
Every limit on your water report is the result of a deliberate process balancing health science against feasibility. Here is how MCLs are set, and how they have tightened over the decades.
When your water report says a contaminant is below its "maximum contaminant level," where does that number come from? Setting drinking water standards is a careful, and sometimes contested, process that balances health science against what treatment can realistically achieve. Understanding it explains why some limits are zero in theory but higher in law, and how the rules have tightened over time.
MCLs and MCLGs
The EPA sets two numbers for each regulated contaminant. The maximum contaminant level goal (MCLG) is the level at which no known health effects occur, set purely on health, with no regard to cost or feasibility. For carcinogens like arsenic and for lead, that goal is zero. The maximum contaminant level (MCL) is the enforceable legal limit, set as close to the goal as is feasible given available treatment and cost. The gap between the two is where science meets practicality.
| Contaminant | MCLG (health goal) | MCL (legal limit) |
|---|---|---|
| Lead | Zero | Action level 10 ppb |
| Arsenic | Zero | 10 ppb |
| Nitrate (as N) | 10 mg/L | 10 mg/L |
| PFOA / PFOS | Zero | 4 ppt |
How a standard gets made
The process runs in stages: the EPA identifies and lists contaminants of concern, studies their health effects and how often they occur, decides whether regulation is warranted, sets the MCLG on health alone, then sets the enforceable MCL considering treatment feasibility and cost, with public comment along the way. It is deliberate by design, which is why new standards can take years from first concern to enforceable rule.
Standards get tighter over time
As science improves and detection gets more sensitive, limits tend to tighten. Arsenic is the clearest example: the old standard of 50 parts per billion, set decades ago, was lowered to 10 ppb in 2001 and fully enforced from 2006, a fivefold reduction reflecting better evidence of cancer risk.
Limits are not all the same
Different contaminants carry very different limits, based on how harmful each is and how it behaves. Comparing a few limits that share the same unit shows the range, from single digits for the most potent substances to higher thresholds for others.
PFAS sit in an even lower range; at 4 parts per trillion, the PFAS limits are roughly a thousand times smaller than the figures above, which is why they required new, highly sensitive testing.
What the trend tells you
Across decades, the direction of travel is clear: more contaminants regulated and limits set lower as evidence accumulates, from the original list under the Safe Drinking Water Act to roughly 90 contaminants today and the recent addition of PFAS. For consumers, the practical takeaway is that "meets the standard" reflects current science and feasibility, not a guarantee of zero risk, which is exactly why the health goal for the most serious contaminants is set at zero. To see where your own water falls against these limits, read your water quality report or test your water.