Our standards & methodology

How we decide what's safe — and why "legal" isn't the same answer.

Plain version: we don't make up our safety levels, and we don't borrow anyone else's. We build them from public health science you can click through to yourself.

What a WaterQualityCheck safety standard is

A WaterQualityCheck safety standard is a health-based level: the amount of a contaminant your water can reach before it crosses a recognized health-protective threshold — set purely on health, with no regard for what's cheap or convenient to treat. To keep every number defensible and independent, we adopt values in this order:

1. California Public Health Goals (PHGs). Health-only goals from California's Office of Environmental Health Hazard Assessment. For cancer-causing chemicals they're set at the one-in-one-million lifetime risk level, and by law cannot consider cost.

2. EPA health goals (MCLGs). The federal health goal for each contaminant. For carcinogens like arsenic, lead and PFOA, this goal is zero. Used where no California PHG exists.

3. Peer-reviewed science. Used only where neither of the above exists yet — for example, the latest research linking nitrate to cancer below the legal limit.

What "above the standard" means — and doesn't

A health goal is not a line between "safe" and "dangerous." It's a protective target. So when we say your water is "58× the standard," we mean it's 58 times above the health-protective goal — not that it will make you sick today. The point is informed choice, not alarm.

Why legal doesn't mean safe

This isn't our opinion. It's how the federal standard is built — in the government's own words.

The gap is intentional

The EPA sets each enforceable legal limit "as close to the health goal as feasible" — explicitly taking cost and available treatment technology into account. The health goal itself considers only health. For cancer-causing contaminants, that health goal is often zero, while the legal limit is set higher because zero isn't considered affordable to reach. Water that meets the legal limit is legal. It is not necessarily without risk.

The cancer math at the legal limit

These lifetime cancer-risk figures come straight from California's official Public Health Goal reports:

Contaminant	Risk at the legal limit	Risk at the health goal
Arsenic	~2.5 cancers per 1,000 people (at the 10 ppb limit)	1 per 1,000,000 (at 0.004 ppb)
Hexavalent chromium	~5 per 10,000 (at the 10 ppb limit)	1 per 1,000,000 (at 0.02 ppb)
Uranium	~5 per 100,000 (at the 20 pCi/L limit)	1 per 1,000,000 (at 0.43 pCi/L)

Many limits are old, and weren't set for the risks we now know about

The clearest example is the one parents care about most. The legal limit for nitrate — 10 mg/L — traces back to a 1962 recommendation aimed only at "blue baby syndrome" in infants, not cancer. Since then, large studies have found increased colorectal-cancer risk far below that limit: a 2.7-million-person Danish study found a significant increase above roughly 0.9 mg/L, a Spain/Italy study around 1.7 mg/L, and a U.S. study found higher cancer mortality even at levels below the legal limit. The legal number hasn't caught up with the science, so we set our nitrate standard at the one-in-a-million lifetime cancer-risk level — about 0.14 mg/L, far below the legal 10.

The standards, contaminant by contaminant

A working summary. Values marked provisional are still being finalized against their specific public source before they appear next to a multiplier.

Contaminant	Legal limit	Our standard	Source
Arsenic	10 ppb	0.004 ppb	CA Public Health Goal (2004)
Hexavalent chromium	10 ppb (CA)	0.02 ppb	CA Public Health Goal (2011)
Lead	15 ppb (action)	0 — no safe level	EPA health goal (MCLG)
PFOA	4 ppt	0.007 ppt	CA Public Health Goal
PFOS	4 ppt	0 (PHG pending)	EPA health goal (MCLG)
Uranium	20 pCi/L	0.43 pCi/L	CA Public Health Goal
Radium	5 pCi/L	0	EPA health goal (MCLG)
Bromate	10 ppb	0.1 ppb	CA Public Health Goal (2009)
Nitrate	10 mg/L	0.14 mg/L	1-in-1,000,000 cancer risk (peer-reviewed)
Trihalomethanes	80 ppb	0.15 ppb	1-in-1,000,000 cancer risk (peer-reviewed)

This is a working draft. Every value will carry its direct citation, and the standards will be independently reviewed, before public launch.

Sources

• U.S. EPA — How EPA Regulates Drinking Water Contaminants
• U.S. EPA — National Primary Drinking Water Regulations
• Congressional Research Service — Regulating Contaminants Under the Safe Drinking Water Act (R46652)
• California OEHHA — Public Health Goals (PHGs)
• Cancer-risk-at-the-limit figures: California utility Public Health Goal reports (citing OEHHA risk assessments)
• Nitrate & cancer below the limit: Schullehner et al. 2018 (Denmark); Espejo-Herrera et al. 2016 (Spain/Italy); Environmental Research 2019; long-term U.S. mortality analysis (PMC)