Synergistic Causation in Tort

**This is an abbreviated version of an article submitted for publication in the Journal of Personal Injury Law in October 2023**


Tim Trotman[1]

Synergy – additive – multiplicative – causation – doubles the risk –material contributionasbestos – smoking – lung cancer


The purpose of this article is to look at synergistic causation in the current law of tort.  Existing references to synergy in case law will be examined, a definition of synergy put forward, and an attempt made to place synergy within the existing landscape of both material contribution, and Fairchild liability. Many examples discussed below derive from the specific case of asbestos/tobacco interaction to cause lung cancer. Mesothelioma is not discussed here, which has its own rules under s.3 of the Compensation Act 2006.



Shortell v Bical Construction Limited [2008] WL2148256 (Mackay J) 

A fairly heavy smoker, the deceased died of lung cancer having been exposed to asbestos by a single defendant. The experts agreed that both asbestos and smoking were risk factors for lung cancer, and that there was “multiplicated” interaction (para 33(iii)) between the two risk factors.  At that time, a North American study (Hammond 1979), suggested a magnitude 10 risk factor for smoking multiplied by a magnitude 5 risk factor for asbestos produced  a magnitude 50 risk of lung cancer. 

Dr Rudd, the Claimant’s respiratory expert chose to use percentages and his opinion was set out by Mackay J. at paragraph 37: 

“… he estimated the deceased’s risk factor from asbestos exposure at 5 fold, acting synergistically upon a 5% risk of lung cancer from smoking, giving a risk from both hazards of 25%.  The synergistic effect of the asbestos has therefore increased his total risk of lung cancer from 5% to 25% which is an increase of 20%. Asbestos as therefore accounted for 20%/25%, i.e. 80% of his total risk…”

The Defendant was prepared to concede that if the risk factor from breach more than doubled the relative risk of contracting lung cancer, the Claimant’s case would be proved.  In fact, Mackay J  accepted an asbestos exposure of 99 fibre/ml years which carried with it on its own a relative risk of lung cancer greater than two. 

It is now understood that the “20% increase” above involves a computational error sometimes called the “Shortell fallacy”.  It has also been questioned whether it was necessary to resort to the concept of  relative risk at all in order to find for the Claimant. 

Novartis Grimsby Limited v Cookson [2007] EWCA Civ 1261

The Claimant had been a moderate cigarette smoker for about 20 years, giving up his habit in 1980.  He had also been exposed to aromatic amines which had been a recognised cause of bladder cancer for many years.  The CA continued at [44] (emphasis added)  :

“Cigarette smoke contains amines and the amines from both sources act on the body in the same way. It was accepted by both sides that the two forms of exposure would have had at least an additive, if not multiplicative, effect. The argument between the parties was about the relative potency of the effects of smoking and occupational exposure. In essence, the argument was whether the occupational exposure was sufficient to have caused or materially contributed to the development of the cancer.

      The Employer argued the Claimant could not rely on an argument of material contribution  : the bladder cancer  was not divisible in the way that pneumoconiosis was divisible. It could not be said that either of the exposures (smoking or amines) made the disease more severe. Therefore it could not be said that either exposure had contributed to the disease. Each had contributed only to the risk that the disease would develop. Respectfully the CA stepped around that problem by finding that amine exposure  had in any event more than doubled the risk due to smoking, although it is not clear whether or not the Shortell fallacy may again have played a role.

 Bailey v Ministry of Defence [2008] EWCA Civ. 388

The well known facts are that the aspiration of vomit (leading to brain damage via a cardiac arrest) was caused by physical weakness of twofold origin, negligent aftercare in hospital, and constitutional pancreatitis.  Again, the interest lies in commentary, here by Professor Sarah C Green in her book “Causation in Negligence” at page 108 (emphasis added): 

“…the potential causal factors in Bailey could only be described as inter-continua dependant because it is not possible to establish what effects negligent aftercare and pancreatitis would have, if suffered separately. Their effects are best described as synergistic, or at least potentially so, meaning that Bailey belongs outside of the duplicative cause category”

Sienkiewicz v Greif UK Limited (SC)E [2011] UKSC10

The Claimant’s mother died of mesothelioma in circumstances that had included both occupational and environmental exposure to asbestos.  Commenting on Shortell above, Lord Phillips at paragraph 75 had no problem with material contribution as an applicable test in cases of asbestos and smoke interaction ( emphasis added): 

“Applying the Bonnington test of causation, the issue in [Shortell] was whether the asbestos to which the victim had been exposed had made a material contribution to the cause of the victim’s lung cancer.  The expert evidence given by both medical and epidemiological  experts…based… on epidemiological data was that asbestos and cigarette smoke not merely combined cumulatively to cause lung cancer, but they had a synergistic effect in doing so.  This evidence was enough as I see it to satisfy the Bonnington test at causation, and the victim had been exposed both to significant quantifies of asbestos fibres and to significant cigarette smoke”

 Jones v Secretary of State for Energy & Climate Change [2012] EWHC 2936 (Swift J.)

Numerous claimants sought damages for respiratory disease and/or various types of cancer from exposure to dust and/or fumes containing cast intergenic substances in the course of work at a plant making carbon fuel brickettes or phurnacite. 

Again, commenting on Shortell, the court, Swift J. stated as follows with echoes of the argument in Novartis above ( emphasis added) : 

“…Multiplicated effect described in Shortell referred not to the biological mechanism which took place in order to cause the cancer, but to the greatly increased effect on risk produced by exposure to a combination of the two carcinogens.  It seems to me that the court in Shortell was really being told not that asbestos and cigarette smoke had combined cumulatively to cause the Claimant’s lung cancer, but that asbestos and cigarette smoke had combined cumulatively to increase the risk of lung cancer. Therefore, it seems to me that the Judge at first instance was right to decide the case on the basis of “doubling of risk”.

More recent research does now tend to look at synergy, certainly in the field of asbestos exposure and smoking, in biological rather than purely statistical terms. Asbestos Smoking and Lung Cancer: an Update International Journal of Environmental Public Health 2020 January 17(1) 258 (Klebe Leigh Henderson et al) a emphasises at 3.4.1 that the interaction between asbestos and smoking “happens at a biological level and is not just a statistical interaction”. 

Blackmore (Executrix of the Estate of Hollow) v The Department for Communities & Local Government [2014]  2YM53819 (HHJ Cotter QC) 

The Deceased, born in 1936 started smoking aged 14 in 1950.  He worked in the Devonport Dockyard between 1966 and 1986 sweeping piles of asbestos fibre without respiratory protection.  Since the age of 14 he smoked 20 cigarettes a day about which habit he received written advice in the mid-1970s.  In October 2010 he died of lung adenocarcinoma. The Defendant admitted liability not on the basis of Fairchild, but on the basis of the doubling of risk principle.  The experts agreed that tobacco smoke and asbestos exposure worked in a synergistic manner in the causation of lung cancer. The form of synergy put forward was frank multiplication.  “Relative Risk” itself was taken as the building block of a calculation to produce a “combined relative risk of lung cancer”



Percentage Risk







Combined Relative Risk of Lung Cancer

19.11 (9.1 x 2.1)



The relevance in the case was that as a matter of pure arithmetic, the figures above seemed to give the Defendant an argument for 88% contributory negligence from smoking. In the event the Court found 30% .

However the figures in the table above can be made to illustrate the aforementioned  Shortell  fallacy. First, assume that asbestos is the background factor against which the addition of smoking is tested.  Smoking would then be responsible for increasing the risk from 2.1 to 19.11.  In other words, smoking on this basis is responsible for 17.01/19.11 of the combined total relative risk over background, or 89.32 % thereof. Asbestos takes the remaining 10.68%. Smoking appears to have more than  doubled the risk of lung cancer.

However, assume smoking as the background factor against which the addition of asbestos is tested.  Then, asbestos will be responsible for increasing the risk from 9.1 to 19.11 over background.  In other words, asbestos is responsible for 10.01/19.11 or 52% of the total increased relative risk over background, with smoking taking 48%.[2] By a lesser margin but sufficiently so, asbestos exposure appears to have more than  doubled the risk of lung cancer.

It is for the above reason that in Blackmore at paragraph 33, HHJ Cotter QC said of Dr Rudd’s analysis in Shortell in paragraph 3 above ( emphasis added): 

“However, the error in this analysis can be most simply shown by the fact that as the respective risks are each 5% and the effect multiplicative the calculation can simply be reversed leading to tobacco being responsible for 80% of the total risk. When using a multiplicated approach, i.e. multiplies two facts to get a product, the relative contribution can only be in line with a ratio and not achieved by simple subtraction

      The Court’s  reference to the need for a ratio is significant and has been explored below.

      Blackmore was appealed (at [2018] PIQR P1) but the Court again rejected a pure risk-based approach to contributory negligence apportionment.

Heneghan v Manchester Dry Docks Ltd and Others [2014] EWHC 4190

In Heneghan at first instance, at paragraph 14, a multiplicative relationship between smoking and asbestos exposure was recognised, but this was treated as supporting the dominant role of asbestos. It was said that the risk of the deceased developing lung cancer at the levels of asbestos exposure experienced were more than five times greater than they would have been had he been just a smoker.  With very great respect it is possible that  the Court may again have embraced the Shortell fallacy although perhaps with inadvertent encouragement by both experts, Dr Rudd and Dr Moore-Gillon.

Otherwise, Mr Justice Jay at paragraph 4 said as regards contributory negligence the issue of smoking had “ fallen away”. This may variously have been a reference to a Barker apportionment where contributory negligence does not arise (later explained in Blackmore 2018PIQR P13 at [35]) or to a finding of fact that smoking had not in fact played any causative role.



To start with, it can be said what synergy is not.  Where there is accepted to be a number of factors which are in competition as the true cause of an outcome, there would seem to be no place by definition for synergy to occur. Hotson v East Berkshire Health Authority [1997] AC750  and Wilsher v Essex Area Health Authority [1988] AC1074 illustrate such  competing factors.

As to what synergy is or may be, it is possible to construct calculations which illustrate synergy.  It is implicit in those calculations immediately below that each factor has the potential to cause the index outcome, e.g. in the case of lung cancer, it is already known that it may be caused by asbestos alone and by smoking alone. To simplify it is assumed below that the factors also act simultaneously, not consecutively.  Synergy would appear prima facie to assume simultaneous action. It is therefore more restricted than other factual scenarios amenable to a material contribution analysis, which can accommodate  successive events :see Williams v Bermuda Hospitals Board [2016] AC904 at para. 38. 

With those caveats imagine Factor X with a potency or (“valence”) of “5” and Factor Y with a valence of 7: 







X acts together with Y




X acts together with Y


supra -additive synergy


X acts together with Y


multiplicative synergy


X acts together with Y


supra -multiplicate synergy


All three outcomes in the table above, that is supra -additive synergy, multiplicative synergy and supra -multiplicative synergy are amenable to the same special approach.  All three would generate a synergy index greater than 1 in the calculation below. 

A formula does exist to detect the quantum of contribution made by a single factor to a synergistic result[3].

S =    XAS – X0

    (XA +XS) – 2 X0




The relative risk for lung cancer among those non-exposed to asbestos and non-smokers


The corresponding value for lung cancer amongst asbestos exposure in non-smokers


The relative risk for lung cancer and smoking in those without asbestos-exposure 


The relative risk for lung cancer and co-exposure to asbestos and smoking


The synergy index and interaction on an additive scale.   S = 1 suggesting no interaction between asbestos exposure and smoking on lung cancer; S ˃ 1 suggests a positive interaction (synergism)


It would be possible to isolate out XA or XS.  XA could be isolated out as follows:-

XAS – X0  = (XA + XS ) – 2 X0


XAS – X0   + 2 X0  – XS  = XA



To isolate out XA in this way and no more might be to fall again into the Shortell fallacy.  As the court said at first instance in Blackmore, relative contributions have to be in line with a ratio,  which ratio could sensibly be as follows:

 XAS – X0

              XA +XS


The ratio would be applied to, say, XA to identify its true valence (XAV) meaning its value as a statement about risk once the effect of synergy has been taken into account : 

XA   x    XAS – X0 = XAV

              XA +XS


          A full worked example is set out in the Appendix to this article.



By way of background it is possible to extract two fundamentally different approaches to the test of material contribution from recent authorities such as :-


Bailey v Ministry of Defence [2008] EWCA Civ. 388

Sienkiewiczg v Greif UK Limited (SC)E [2011] UKSC10

Jones v Secretary of State for Energy & Climate Change [2012] EWHC 2936

Heneghan v Manchester Dry Docks Ltd and Others [2014] EWHC 4190.

Heneghan v Manchester Dry Docks Ltd [2016] EWCA Civ 86.

Williams v Bermuda Hospitals Board [2016] UKPC 4.

Thorley v Sandwell and West Birmingham Hospital NHS Trust [2021] EWHC 2604.

Davis v Frimley Health Foundation Trust [2021] EWHC 169.


In the first approach the tendency is to confine the role of material contribution to cases of divisible injury : 

This approach reached its apogee in the CA decision in Heneghan where Lord Dyson MR at paragraph 23 gave some examples of material contribution but continued  by way of distinction (emphasis added)  “thirdly where causation cannot be proved in either of these ways, for example because the disease is indivisible”. 

Confining the test of material contribution to divisible injuries can be traced back to AB v Ministry of Defence[2010] EWCA Civ 1317. 

The same approach is also one which tends to elevate the value of the test of doubling of risk, a test founded it is said on orthodox principles of causation : see Novartis above, Lord Dyson in Heneghan (CA)  , and Lord Justice Lloyd Jones in Blackmore [2017] EWCA Civ 1136 [34]. Lord Dyson said in Heneghan (CA)  [2016] 1 WLR at 2040  F-G:


The doubles the risk test is one that applies epidemiological data to determine causation on the balance of probabilities where medical science does not permit determination with certainty of how an injury was caused.  If statistical evidence shows that a tort feasor more than doubled the risk that the victim would suffer the injury it follows that it is more likely than not that the tort feasor caused the injury”.


In reaching these views Lord Dyson was also confirming a similar approach in the Heneghan decision below. There  Mr Justice Jay had rejected Dr Rudd’s deterministic approach and held ( at para 30) that it did not follow from the proposition that because 2.5% of the dose may be attributed (to the Fourth Defendant) ,  that such a Defendant had materially contributed to the  Claimant’s carcinoma.

Swift J’s endorsement of the approach of Mackay J in Shortell in her decision in Jones in paragraphs 11 to 13 above falls into the same approach : indeed she ultimately rejected material contribution as a useful test in Jones :

[655] All these considerations lead me to the conclusion that it cannot be right to approach the cases of lung cancer — nor indeed those of bladder cancer — by applying the Bonnington principle. …….


Soole J in Thorley v Sandwell and West Birmingham Hospital NHS Trust [2021] EWHC 2604 followed AB v Ministry of Defence[2010] EWCA Civ 1317 and Heneghan v Manchester Dry Docks Ltd [2016] EWCA Civ 86 as a matter of strict precedent [151].


Under the second or alternative approach the tendency is to grant to the test of material contribution a much broader application :

The second or alternative approach clearly has its origins in the broad application of a test of material contribution in Bailey  ; 

The line of the second or alternative  approach goes through  Lord Philips’ dictum  in Sienkiewicz v Greif UK Limited (SC)E  at paragraph 90 as to full liability, which is also less enthusiastic about the doubles-the-risk test : 

“90.  For reasons that I have already explained, I see no scope for the application of the “doubles the risk” test in cases where two agents have operated cumulatively and simultaneously in causing the onset of a disease.  In such a case, the rule in Bonnington applies where the disease is indivisible such as lung cancer.  A defendant who has tortiously contributed to the causes of the disease will be liable in full”


The approach received some endorsement in Williams v Bermuda Hospital where at paragraph 32 the Court noted that in  Bonnington there was no suggestion that the pneumoconiosis was “divisible” ;  rather Lord Reid in that case had interpreted the medical evidence as meaning that the particles from the swing grinders were the cause of the entire disease.  Lord Toulson in Williams added:-“True they were only part of the cause, but they were a partial cause of the entire injury as distinct from being a cause of only part of the injury”.

Financial Conduct Authority v Arch Insurance [2021] AC 649 is likely to fall within this group.


Synergy : First and Second Approaches above compared

How does synergy fare as an analytical device under the first or the second approach above ? 

The first approach is one where a Court is more likely to look at an indivisible injury as being the product only of combined risks : see again Swift J in Jones  who described asbestos and cigarette smoke  combining cumulatively to increase the risk of lung cancer ; and Novartis where it was argued amines and smoke contributed only to risk. However it cannot be ignored that (if such be the case) the agents producing risk have also reacted together synergistically in doing so. The need to avoid the Shortell fallacy may then prevent any or any customary application of the doubles the risk test although that would often otherwise be the preferred device where the first approach is adopted ( sub-para (c) above).  If the ratio method is adopted (see Appendix) of extrapolating base risks in a synergistic outcome, the final values again do not easily lend themselves to a doubles-the-risk analysis.  This is so even though the final values have their origins in base figures -  XA and  XS   - which were themselves statements of relative risk.

If the second approach applies with its emphasis on a broader application of the material contribution test , will a defendant responsible for one of the synergistic factors always be liable for 100% of  an indivisible loss? Such factor, it will be said, will always, by synergy, be a partial cause of the entire injury as distinct from being a cause of only part of the injury.  Is such an outcome fair where synergy has inevitably magnified the initial input, perhaps bringing that initial  input out of “de minimis” ? Swift J had such unfairness in mind more generally in Jones  where she said at para 655

…………….Moreover, to adopt the claimants’ arguments would……have potentially far-reaching effects. It would mean that, in any case of cancer where a claimant could establish tortious exposure to a carcinogen that was ‘material’ (according to whatever measure of materiality the court chose to adopt) the claimant would succeed in establishing causation and would be entitled to 100% damages. ……I cannot accept that such a result would be fair to potential defendants who would be required to pay full damages in many cases in which occupational exposure had played a small part or, perhaps, no part at all.

The problem above may be less acute where the dispute is a basic one of the kind described in Amaca above : did exposure to the given reagent have any harmful effect on the Claimant? There it could be valuable for a Claimant to point to the likelihood of synergistic interaction in response to the question. But where the issue is which of a number of exposing Defendants should be responsible for loss, Swift J’s point respectfully retains force.

Could the egg shell skull rule perhaps help, allowing a small amount of asbestos exposure to make an acceptably big impact on a background weakness attributable to smoking ? But the rule is a  principle of remoteness of loss,  rather than causation . A sounder basis of liability upon a defendant responsible for one of the synergistic factors would be  the principle in Hughes v Lord Advocate [1963] AC 837 applied in ROH v Goldscheider [2019] EWCA  Civ 711 and Chadwick v Ovenden 2022 EWHC 1701 QB  ( where the court reiterated Lord Guest at P-714-D in Hughes that it was sufficient if the accident which occurred was of a type which should have been foreseen by a reasonably careful person.) A defendant, if in breach, cannot complain, under this principle, that the manner of loss-occurrence is unforeseeable (here the obscure or stochastic nature of a synergistic process.)  And that  should overcome Swift J’s objection about unfairness.


One conventional requirement of the Fairchild exception is that the causative factors under scrutiny must all have acted in the same way- cf Barker v Corus UK Ltd [2006] 2 AC 587 (“Barker”) : That there is such a requirement of action in the same way was emphasised in Sienkiewicz at 104. 

So a dichotomy develops between agents acting in the same way and amenable to Fairchild  analysis ; and agents acting in different ways not amenable to that analysis (which latter category appears to include asbestos and smoke). A requirement that all factors act in the same way would at first blush appear to exclude synergy, which logically arises from two or more factors each acting in different, albeit inter-acting, ways. Swift J elucidated this point in Jones at paragraph 627 (emphasis added):

“Professor Jones did not accept that the operative carcinogens contained in cigarette smoke and in the dust and fumes to which workers at the furnace site plant were exposed were necessarily the same.  He said that both tobacco smoke and the dust and fume produced at the furnace site plant contained a large number of different carcinogens. It could not be assumed that BAP had been the only operative substance in each case.  If it had been, its effect would not have been multiplicated (or nearly so) as he and Dr Rudd had agreed.  Instead, it would have been additives

However in  Heneghan v Manchester Dry Docks Ltd [2016] EWCA Civ 86 Lord Dyson MR confirmed that the Fairchild principle was applicable to lung cancer cases. Thus a  hitherto relatively straightforward picture has been altered by the formal admission into the Fairchild enclave of lung cancer post Heneghan litigation, where inevitably smoking will in many cases have played a causative role alongside asbestos.  There is now a difficult overlap between looking at asbestos and smoking from the synergistic perspective, and from the perspective of contributory negligence. As stated the conventional approach to Fairchild  liability in lung cancer cases post -Blackmore 2018 PIQR P1 is that contributory negligence does not arise, but it has been argued that this is an oversimplification : Asbestos Law & Litigation 2nd ed  Sweet & Maxwell 12-025.  Whatever the correct approach, it is suggested that synergy comes first as it must be confronted in order to identify what exactly is the contribution asbestos has made.

As to the Barker  risk apportionment exercise (which continues to apply in lung cancer cases) a time-of-defendant-exposure basis was referred to at [2006] 2 AC 48 ( Lord Hoffmann) and again at [2006] 2 AC 62 (Lord Scott). Both statements  also admitted the intensity of exposure and exact type of asbestos. The risk apportionment exercise is an obvious question of fact for the Court, although now to be viewed through the prism of the “weak” or “broad” causal link identified in Durham v BAI [2012] UKSC 1. It must have relevance to the question of fact that the effect of asbestos exposure was, say at certain times, subject to synergistic inter-action with cigarette smoke, and so may have had a greater effect than a strict time apportionment would allow.

Further the Barker risk apportionment  exercise  as understood in Blackmore 2018 PIQR P1  assumes that total asbestos contribution and smoking contribution can be logically separated. This goes back, as stated, to the identification of several liability, for example by Lord Hoffmann in Barker at [47]. If so, the ratio method in the Appendix  might provide a technique to isolate out the total effect of asbestos exposure, after which apportionment of that total can take place as needs be between the exposure periods of different defendant employers.



[1]Deans Court Chambers 24 St John Street  Manchester  M3 4DF


[2] There is a very clear exposition of the fallacy in “Asbestos Lung Cancers – where are we now after Blackmore ?”.  Rawlinson KC  12 Kings Bench Walk 17 08 17 

[3] Additive Synergism between Asbestos and Smoking in Lung Cancer Risk: a Systematic Review and Meta-analysis Ngan Wong 14.08.15

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Tim trotman