Quantifying Material Increase in Mesothelioma Risk: Direct Risk Assessment in Johnstone v Fawcett’s Garage

1. Introduction

The Court of Appeal’s decision in Johnstone v Fawcett’s Garage (Newbury) Ltd [2025] EWCA Civ 467 addresses how courts should quantify and assess causation in asbestos‐related mesothelioma cases. Mrs Elaine Johnstone died of malignant mesothelioma in 2019 after years of office employment adjacent to a garage workshop where asbestos dust was generated. Her husband, Mr Alexander Johnstone (the appellant), sued the garage (the respondent), alleging that unsafe asbestos‐handling practices materially increased his wife’s risk of developing the disease. The garage admitted breach of duty but disputed that its negligence caused a material increase in risk. At trial, the judge quantified occupational and environmental exposures and concluded the incremental risk was 0.1% or less—de minimis. The Court of Appeal dismissed five grounds of appeal, clarifying the proper methodologies for assessing risk and rejecting adverse inferences and “absolute risk” approaches.

2. Summary of the Judgment

The trial judge carried out a three-stage exercise:
- Stage 1: Calculate the workshop mechanic’s (RR’s) asbestos exposure—found to be approx. 1 f/ml-y or less.
- Stage 2: Infer Mrs Johnstone’s exposure by applying distance, time and reduction factors—estimated at 0.001–0.002 f/ml-y.
- Stage 3: Convert exposure into an increase in mesothelioma risk using epidemiological models—concluding the increase was 0.1% or less.
The judge preferred the occupational hygiene evidence of Mr Stear over Mr Chambers and the toxicology and epidemiology of Professor Jones over Professor Norrie, finding Stear’s and Jones’s methodologies aligned more closely with the facts.
Five grounds of appeal were rejected:
- Ground 5: No adverse inference from missing air‐monitoring records—no principle required uncritical acceptance of the appellant’s higher figures.
- Grounds 1–4: The judge’s treatment of expert evidence, choice of “direct risk assessment” over the “exposure/risk‐correlation” method (Method 1), and refusal to adopt an “absolute medical significance” test were all upheld.
The Court affirmed that an increase in risk of 0.1% or less is de minimis and does not meet the special causation test for mesothelioma.

3. Analysis

3.1. Precedents Cited

Fairchild v Glenhaven [2003] 1 AC 32: Established the special rule that a mesothelioma claimant need only show a material increase in risk, not but-for causation, where scientific uncertainty prevents traditional proof.
Sienkiewicz v Grief [2011] 2 AC 229: Extended Fairchild to single tortfeasors and emphasized judges must decide materiality on the facts, not by fixed quantitative thresholds.
McGhee v National Coal Board [1973] 1 WLR 1: Introduced the concept that a breach of duty may give rise to liability if it materially contributed to risk where precise causation is unknowable.
British Railways Board v Herrington [1972] AC 877 and subsequent authority (e.g., Keefe and Shawe-Lincoln): Provided the framework for drawing adverse inferences from missing evidence.
Mackenzie v Alcoa [2019] EWCA Civ 2110: Confirmed that whether to draw an adverse inference depends on the facts, including the explanation for non-production.

3.2. Legal Reasoning

The judgment’s core legal framework is the special causation rule for mesothelioma: liability arises if the defendant’s breach materially increases the claimant’s risk. The judge’s reasoning proceeded by:

Quantifying RR’s and Mrs Johnstone’s asbestos exposures on a fibres-per-millilitre-per-year (f/ml-y) basis.
Reviewing three competing methodologies:
1. Exposure/Risk-Correlation Approach: Divide occupational exposure by lifetime environmental exposure to derive percentage increase in risk.
2. Direct Risk Assessment Approach: Use Hodgson & Darnton’s meta-analysis to extrapolate risk from known exposure, then compare against background risk (Peto Report).
3. Absolute Risk Approach: Ask a medical expert whether the incremental risk is “medically insignificant.”
The judge adopted the direct risk assessment approach, finding:
- Occupational risk at most 0.022 deaths per 100,000 (best estimate).
- Background risk approx. 0.5 per 1,000 (0.5 ‰), yielding 50 per 100,000.
- Percentage increase = 0.022 ÷ 50 × 100 = 0.044% (rounded to “0.1% or less”).
The Court of Appeal confirmed that the judge was entitled to prefer the evidence of Mr Stear and Professor Jones and to reject the adverse-inference submission and the absolute risk approach as inconsistent with the special rule.

3.3. Impact on Future Cases and the Law of Tort

This decision clarifies several points for mesothelioma litigation:

Courts may adopt a “direct risk assessment” methodology, using vetted epidemiological models (e.g., Hodgson & Darnton) with appropriate caution.
Judges remain fact-finders on expert disputes over exposure calculations and model reliability; appellate interference requires a conclusion that no reasonable judge could have made those findings.
Adverse inferences for missing safety records are not automatic remedies where expert interpretation of witness evidence is central.
The special rule requires comparative risk analysis, not a standalone medical judgment of “insignificance.”
No fixed quantitative “materiality” threshold exists; increases as low as 0.1% may be de minimis, but courts must assess materiality case by case.

4. Complex Concepts Simplified

f/ml-y (fibres-per-millilitre-per-year): A metric combining asbestos concentration in air and duration of exposure (e.g., 1 f/ml for 1 year = 1 f/ml-y).
Background vs Occupational Exposure: Lifetime environmental exposure (homes, schools) versus specific workplace exposure under defendant’s control.
Material Increase in Risk: A non-de minimis uplift in mesothelioma risk due to defendant’s breach, assessed comparatively.
De Minimis: So insignificant that the law “properly disregards” it; no fixed numerical cutoff, left to judicial judgment.
Action Levels & Control Limits: Regulatory thresholds (e.g., 0.5 f/ml over 4 hours) triggering monitoring and record-keeping under the Asbestos Regulations.
Adverse Inference: Drawing negative inferences from a party’s failure to produce evidence, applied only where the missing evidence would have been central and was withheld without satisfactory explanation.

5. Conclusion

Johnstone v Fawcett’s Garage reaffirms and refines the Fairchild rule for mesothelioma claims. It confirms that:

Causation may be established by demonstrating a material increase in risk, not but-for proof.
Direct risk assessment—using cautious extrapolation from high-dose studies and comparison with background risk—is a valid methodology when grounded in sound expert evidence.
The trial judge’s role as fact-finder in choosing between competing expert models is sacrosanct absent a plainly wrong conclusion.
Missing regulatory records do not automatically lead to uncritical acceptance of higher exposure figures.
An “absolute risk” or purely medical significance test would undermine the risk-based causation rule.

As a result, future asbestos litigation will continue to focus on precise exposure quantification, careful use of epidemiological models, and holistic judicial evaluation of expert evidence to determine whether a defendant’s breach materially increased a claimant’s mesothelioma risk.