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Office-led project

Adoption of the Fixed Concentration Procedure for acute inhalation studies

At a glance

In progress
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  • Reduction
  • Refinement


On 9 October 2017, a new and refined test guideline for acute inhalation studies was officially adopted by the OECD: Test No. 433: Acute Inhalation Toxicity: Fixed Concentration Procedure (FCP).  This was the result of an initiative that we led, working with inhalation toxicity experts, to support adoption of the FCP.  This test used fewer animals than existing test guidelines and does not require lethality as an endpoint – giving the FCP clear animal welfare benefits. 

The principle underlying the FCP is the identification of ‘evident toxicity’, that is clear signs of toxicity such that it can be predicted that exposure to the next highest concentration would cause severe toxicity or death in most animals.  Testing at the higher concentration can then be avoided.

The FCP was dropped from the OECD work plan in 2007 because of a lack of evidence for comparable performance with existing test guideline methods, suspected sex differences in toxicities (FCP originally only used females) and the subjective nature of evident toxicity. 

In 2008 the NC3Rs began a three-stage strategy of work to address these issues. We initially commissioned research to compare the FCP with existing methods and found that, in the absence of sex differences, the FCP is comparable.  An additional pilot study has been incorporated to account for any potential sex differences. This work has been published in Human and Experimental Toxicology [4, 5].

We then worked with inhalation toxicity experts to address the last concern – the subjective nature of evident toxicity.  We collected data to provide objective guidance on the recognition of evident toxicity, in order to make this decision less subjective. 

We developed a clinical signs recording system for signs observed during acute inhalation studies which takes into account the signs observed, their severity and duration. An international trial tested the usability of the recording system. It has also been applied to historical data on the clinical signs observed for 188 substances from 511 acute inhalation studies provided by seven laboratories worldwide (US, EU, Korea and Japan). This work has been published in Regulatory Toxicology and Pharmacology [1, 3]. 

As a result of evidence provided by this initiative, the FCP was finally adopted by the OECD in 2017. 



  1. Sewell F et al. (2018). An evaluation of the fixed concentration procedure for assessment of acute inhalation toxicity. Regulatory Toxicology and Pharmacology 94: 22-32. doi: 10.1016/j.yrtph.2018.01.001
  2. Poster presented at the NC3Rs Toxicology Showcase, September 2016: A global initiative to refine acute inhalation studies through the use of ‘evident toxicity’ as an endpoint: towards adoption of the Fixed Concentration Procedure
  3. Sewell F et al. (2015) A global inititative to refine acute inhalation studies through the use of 'evident toxicity' as an endpoint: Towards adoption of the fixed concentration procedure. Regulatory Toxicology and Pharmacology 73(3):770-9. doi: 10.1016/j.yrtph.2015.10.018
  4. Price C et al. (2011) A statistical evaluation of the effects of sex differences in assessment of acute inhalation toxicity. Human and Experimental Toxicology 30: 217-238. doi: 10.1177/0960327110370982
  5. Stallard N et al. (2011) A new sighting study for the fixed concentration procedure to allow for gender differences. Human and Experimental Toxicology 30: 239-249. doi: 10.1177/0960327110370983