The record
FDA's contract laboratory established, over more than 200 samples, that TEM/EDS/SAED can detect one asbestos fiber per 0.1 micrograms of talc — corresponding to roughly 10^7 fibers per gram, or approximately 10^-5 percent asbestos by weight. FDA proposed treating detection of a single fiber as a positive result, replacing its prior use of the limit of quantification.
Claim scope
The precise sensitivity gap between the methods: TEM at ~10^-5 percent by weight versus the J4-1 XRD screen at 0.5 percent by weight — roughly four to five orders of magnitude. Also that FDA proposed shifting the positivity criterion from limit of quantification to limit of detection: 'we are proposing to use the limit of detection corresponding to detection of a single asbestos fiber as the basis for a positive sample.' The proposed rule would require examining 'an area containing at least 0.1 micrograms of talc on a TEM grid.' FDA gives the minimum routinely characterizable fiber width by TEM/EDS/SAED as 'on the order of 0.04 [mu]m, corresponding to the typical width of single chrysotile fibrils.'
What this source does not establish
The 10^7 fibers/gram and 10^-5 percent figures are FDA's ESTIMATES ('an estimated', 'approximately'), tied to the 0.1 microgram analyzed mass — not universal constants. Reproduce them with FDA's own hedging words. This does NOT establish that TEM detects all asbestos present; it establishes a stated detection limit, below which 'not detected' still does not mean 'not present.' Also: the single-fiber positivity criterion was PROPOSED, and the proposal was withdrawn — it is not the operative standard.
Limitations
Report verbatim with the exponents intact. Note that FDA's own historical positives were determined using the limit of quantification, not the single-fiber criterion — the proposed change would have made the criterion stricter than the one under which the FY2019 results were generated.
Source
Federal Register — 89 FR 105490 (Dec. 27, 2024), FR Doc. 2024-30544 (Ref. 19), proposed 21 CFR 730.3(c)(1)(ii) and 730.3(d)