A wide spectrum of risk assessors are enamoured of the TTC, which essentially uses a knowledge of the toxicological properties of the universe of tested chemicals to make conservative predictions of the likely toxicity of untested materials. It gives them pragmatic respite from the onward march of analytical chemistry (ever-increasing instrument sensitivity resulting in ever-decreasing limits of detection). Unfortunately, this relief is not shared by those in society who obsessively fear a contaminated food supply and environment. So, does the science supporting TTC justify the regulators’ confidence or are the worried right to be concerned?

Oral toxicity TTC

John Frawley, working on food-packaging materials for US company Hercules in the 1960s, was probably the first to propose that there might be a generic human no-effect level that could be used to establish the safety of compounds that have not yet been subject to toxicological scrutiny. He collated and assessed non-cancer data on 220 chemicals from 2-year laboratory animal studies. Of the 132 chemicals that remained after eliminating heavy metals and pesticides, only one elicited toxicity at concentrations below 100 ppm in the diet, this exception being acrylamide which had no effects at 10 ppm. To this 10 ppm, Frawley applied a 100-fold Safety Factor (10 for interspecies uncertainty, 10 for intraspecies differences), and concluded that 0.1 ppm would be the safe dietary concentration for any food-packaging material. Assuming the average US adult consumes 3 kg of food and drink per day, this would equate to an acceptable intake of 300 µg/day.

Some decades later, inspired by Frawley’s work on chronic toxicity, Munro et al. (1996) used data from sub-chronic, chronic and reproduction/teratology studies in rats, mice or rabbits to compile a reference database of oral toxicity data on a wide range of commonly used organic chemicals. The 613 compounds were grouped into three classes according to chemical structure, based on the work of Cramer et al. (1978), i.e. Class I (the least toxic), II, and III (the most toxic). For each chemical, the most conservative no-observed-effect level (NOEL) was selected, based on the most sensitive species, sex and end-point, and was then standardised to a life-time (chronic) value. The 5th percentile NOEL values were 3, 0.9 and 0.15 mg/kg bw/day for Classes I, II and III respectively. Dividing by a safety factor of 100 (again, to take account of possible inter- and intra- species differences) generated respective ‘safe’ human exposure thresholds (for a 60-kg person) of 1800, 540 and 90 µg/day (i.e. 30, 9 and 1.5 µg/kg bw/day) – toxicity TTC values that, rather remarkably, are still in regular use over 20 years later.

Oral cancer TTC

As reassuring as Frawley’s original work was, it did not address the “elephant in the room” – carcinogenicity. Work was undertaken to tackle this, chiefly by the US FDA, under a ‘Threshold of Regulation’ (TOR) initiative. This aimed to set a universally applicable human benchmark that was likely to pose only a low and tolerable cancer risk should any untested chemical present at such a level be subsequently found to possess carcinogenic potential. Taking the cancer potencies of 477 known carcinogens comprising the Gold (Cancer Potency) Database, mathematical modelling allowed the derivation of Virtually Safe Doses (VSDs) for each – daily human doses that will not pose a life-time cancer risk greater than 1 in a million. From these values a generic VSD of 0.5 ppb was derived (Flamm et al., 1987; Rulis, 1989, 1992), equivalent to 1.5 µg/day, the TOR (Munro et al., 1996).

Building upon an FDA study (Cheeseman et al., 1999), a task force of the International Life Sciences Institute (ILSI) examined a dataset of 730 carcinogens to see if more substantial support could be given to the 1.5 µg/day figure (Kroes et al., 2004). Dividing the 730 compounds into 18 separate groups based on structure, the task force identified five of these groups for which the carcinogenic risk would likely be much greater than 1 in a million at the TOR for nearly all qualifying members. These were termed the “the Cohorts of Concern” and – spoiler alert – will be discussed further in the next post. Even when these five potent carcinogenic tribes were excluded, too high a proportion of the members of the other 13 groups still posed a cancer risk at the TOR that exceeded the 1-in-a-million benchmark. This was not the case at a daily exposure ten-fold less than the TOR, hence the birth of the cancer TTC of 0.15 µg/day.

The history of the TTC approach is complex, both in terms of evolution and timescale. What has been presented here is only a brief overview. The references given below provide more detailed reading.

In the next blog post, we will be examining the suitability of the TTC approach for certain chemicals or groups of chemicals. In the meantime, the bibra website contains useful information on how the TTC approach may be applicable to your sector. Our friendly and knowledgeable toxicologists are always willing to provide advice and assistance should you wish to contact us.


Ashby J and Tennant RW (1991). Definitive relationship among chemical structure, carcinogenicity and mutagenicity for 301 chemicals tested by the U.S. NTP. Mutation Research, 257, 229-306.

Cheeseman MA, Machuga EJ and Bailey AB (1999). A tiered approach to threshold of regulation. Food and Chemical Toxicology, 37, 387-412.

Cramer GM, Ford RA and Hall RL (1978). Estimation of toxic hazard – a decision tree approach. Food and Cosmetic Toxicology, 16, 255-276.

Flamm WG, Lake LR, Lorentzen RJ, Rulis AM, Schwartz PS and Troxell TC (1987). Carcinogenic potencies and establishment of a threshold of regulation for food contact substances. In: Contemporary issues in risk assessment. Volume 2. De minimis risk. Edited by D Whipple. pp 97-92. Plenum Press (cited in Cheeseman et al., 1999).

Frawley JP (1967). BIBRA Annual Scientific Meeting. Scientific evidence and common sense as a basis for food-packaging regulations. Food and Cosmetics Toxicology, 5, 293-308.

Kroes R, Renwick AG, Cheeseman M, Kleiner J, Mangelsdorf I, Piersma A, Schilter B, Schlatter J, van Schothorst F, Vos JG and Wurtzen G (2004). Structure-based Thresholds of Toxicological Concern (TTC): Guidance for application to substances present at low levels in the diet. Food and Chemical Toxicology, 42, 65-83.

Munro IC, Ford RA, Kennepohl E and Sprenger JG (1996). Correlation of a structural class with no observed-effect levels: a proposal for establishing a threshold of concern. Food and Chemical Toxicology, 34, 829-867.

Rulis A (1989). Establishing a threshold of regulation. In: Risk assessment in setting national priorities. Edited by J Bonin and D Stevenson. Plenum. pp 271-278.

Rulis RM (1992). Threshold of regulation: options for handling minimal risk situations. In: JW Finley, SF Robinson and DJ Armstrong (Editors). Food Safety Assessment. American Chemical Society Symposium Series, 484, pp 132-139 (cited in Felter et al., 2009).

WHO / EFSA (2016). European Food Safety Authority and World Health Organization. Review of the Threshold of Toxicological Concern (TTC) approach and development of new TTC decision tree. EFSA Supporting publication 2016:EN-1006. Available at http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2016.EN-1006/epdf



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