# Excerpt — Kelly, "The Myth of 10⁻⁶ as a Definition of Acceptable Risk"

*Kathryn E. Kelly, Dr.P.H. (Delta Toxicology). Originally A&WMA 84th Annual Meeting, Vancouver, 1991; the held
copy is the author's **updated** version (post-1998 — carries a 775 area code). The authoritative investigation
of where the 10⁻⁶ "one in a million" acceptable-cancer-risk number came from: Kelly phone-surveyed the federal
"Who's Who" who use it; none could document its origin. **Operator-supplied via Taildrop 2026-06-11; held &
OCR-verified** in `papers/lateral/kelly1991_myth-of-10-6-acceptable-risk.pdf` (+`.txt`). The paper read
firsthand → **[CONFIRMED-primary]** as a source; its claims about 10⁻⁶'s origin are an authoritative
**[secondary]** investigation; the officials' quotes are quotes Kelly collected (attribute to Kelly).*

## The officials' confessions (Kelly's survey of who uses the number)
> "good theories, but no written documentation. A sample of the responses:
> • 'My mind is a complete blank.' • 'I think it came from pesticides legislation or the Delaney Clause.' • 'It
> came from the FDA in the 1950s.' • 'It's based on the chance of being hit by lightning, which is one in a
> million.' • '**I just assumed it was because one-in-a-million sounded like such a nice phrase.**' • 'It was
> selected because it was "**doable**." Or at least that's what we thought at the time.' • 'It was a purely
> political decision made by several of the major agencies behind closed doors in the 1970s. I doubt very much
> you'll get anyone to talk to you about it.' • And our favorite, '**You really shouldn't be asking these
> questions**' (this from one of the federal agencies)."

## The thesis
> "The use of 10⁻⁶ as a definition of acceptable risk thus has **no scientific or regulatory basis. Its use
> appears to be arbitrary** and generally applied where risks are perceived to be high relative to other risks,
> regardless of the available data."

## The origin chain — independently corroborates Rodricks (register H1) and Mantel (H11)
Kelly traces it the same way the held memoir (Rodricks 2019) and the held 1961 paper (Mantel & Bryan) do:
1. **1959 Thanksgiving cranberry scare** (aminotriazole) → HEW asks NCI to say which carcinogens are "safe" and
   at what level.
2. **Mantel & Bryan (1961)** stipulate "safe" = a risk of **1 in 100 million (10⁻⁸)**. Kelly: *"Asked how he
   chose the number of one in one hundred million, **Mantel replied, 'We just pulled it out of a hat.'**"* (This
   is the source of the anecdote — now read firsthand; still **[secondary]**, attribute to Kelly.)
3. **The 1973 "Sensitivity of the Method" Federal Register notice** (carcinogenic animal drugs under the
   Delaney Clause; prompted by **DES** in cattle) adopted Mantel's **10⁻⁸**…
4. …and FDA **"changed this value to 1 in 1,000,000 by the time the final rule was issued in 1977,"** making
   "one in one million" the "maximum lifetime risk that is essentially zero." (Dating of the 10⁻⁸→10⁻⁶ shift =
   **[secondary]** per Kelly; the comprehensive SOM procedural final rule is **52 FR 49572 (1987)** — the FR
   pages themselves remain to be pulled, `papers/lateral/_WISHLIST.md` / `10` §6.) Kelly also notes the de
   minimis maxim verbatim: *"de minimis non curat lex: the law does not concern itself with trifles."*

## Why it matters
This is the essay's *"number they pulled out of a hat"* block in one source: the officials who run U.S. cancer
regulation on 10⁻⁶ cannot say where it came from, and its statistical father called his own input arbitrary.
The carcinogen-side twin of Frawley's 0.1 ppm, confessed. **Links:** [[frawley-dossier-project]] · pairs with
`MantelBryan_1961_virtually-safe-dose_excerpt.md`, `Rodricks2019-Crump2018_10-6-origin_excerpt.md`; ties to
`10` §3.5 and `reckoningscience/sections.md` §9 → the candidate "one in a million" block.