Toxicology basics · Concept 8 of 9 · 3 min

Cancer and non-cancer

Two ways agencies draw the line. Most effects get a daily limit; cancer often gets a straight line and an extra lifetime chance.

"Can cause cancer" and "will cause cancer at your dose" are different claims (my example).

The idea

For effects other than cancer, agencies set a daily limit, the reference dose, meant to cover people over a lifetime, sensitive groups included US EPA, 2026. When the evidence shows a threshold, the curve is drawn flat at low doses US EPA, 2026. For cancer, when the way a substance works isn't known, the default is a straight line from zero US EPA, 2026. The slope of that line turns a dose into an extra lifetime cancer risk US EPA, 2026. So a non-cancer limit asks "is this under the line?", and a cancer number asks "how much extra risk?" (my example)

Cancer and non-cancerTwo lines against dose. The first stays flat at zero until a threshold, then rises; a daily limit sits below the threshold. The second rises in a straight line from zero, the default for cancer when the mechanism is not known.thresholddaily limitcancer default:straight from zeronon-cancer:flat, then risingDose →Response
Two lines: one flat until a threshold, then rising; one rising straight from zero (my example).

Use it on a case

When a label or report mentions cancer:

  1. Ask whether it's about hazard (what a substance can do at some dose) or risk (how likely harm is at your dose).
  2. Look for the dose behind the number, as always.
  3. For a cancer number, ask for the extra risk and over what time, usually a lifetime.
  4. Read the straight line as a default for when the mechanism isn't known, not a measurement at tiny doses.

An example

A label warns that an ingredient can cause cancer (my example). That describes the substance, not your dose (my example). The useful next question is how much reaches you, and how that compares with agency limits (my example).

What it can't tell you

Agency cancer numbers are estimates for groups over a lifetime, not a forecast for one person (my example).

Sources for this page

  • US Environmental Protection Agency (2026). Conducting a human health risk assessment.

    Health agency advice: what a public health body recommends; check the page for its own sources.

    What we use: How agencies go from dose to limit: the NOAEL is the highest tested dose with no meaningful increase in harm; the LOAEL is the lowest dose that showed one. A daily reference dose is the NOAEL divided by uncertainty factors, usually 10 for animal-to-human and 10 for differences between people (100 together). Effects with a threshold get a curve that is flat at low doses; for cancer, a straight line from zero is the default when the mechanism isn't known, and its slope gives the extra lifetime chance. Durations: acute (hours to a day), subchronic (weeks or months), chronic (a large part of a lifetime; for people, at least seven years). Limits aim to cover sensitive groups (for example asthma or genetics) and high-end exposure, around the 90th percentile.

    Checked Oct 13, 2026 · https://www.epa.gov/risk/conducting-human-health-risk-assessment

Every source on the site