V suffered serious brain damage following respiratory failure & died at D's hospital, doctor did not attend (due to technology failure) until after V died
P argued doctor should have attended & carried out a specific procedure, which would have saved V's life
doctor testified she would not have carried out procedure even if she had attended & her evidence was backed by medical professionals
how could did the but for test apply?
D's negligence was based an omission to act, therefore, the court had to consider but for test in hypothetical situation
courts are cautious about finding against medical professionals for policy reasons: may affect willingness to treat patients or try pioneering new procedures & cost of medical care would increase due to higher insurance premiums
Factual causation: multiple causes
if several possible alternative causes C must show that harm was caused by D's breach (as in Wilsher v Essex Area Health Authority )
however, cases often involve harm which may have been caused by a combination of a number of factors
Material contribution approach
material contribution approach used to help determine causation where multiple causes contributed to C's harm
Waller LJ: .. In a case where medical science cannot establish the probability that 'but for' an act of negligence the injury would not have happened but can establish that the contribution of the negligent cause was more than negligible, the 'but for' test is modified, and the claimant will succeed...
P contracted dermatitis due to exposure to dust, when cleaning brick kilns for D
evidence suggested only way to avoid was thorough washing immediately after contact & D negligently did not provide washing facilities on site
however, evidence did not establish whether lack of washing (D was liable for) or more generally exposure (D not liable for) was the cause
was sufficient causation proved?
P could not prove D caused his dermatitis under strict all or nothing approach(Hotson v East Berkshire Area Health Authority )
P also unable to prove D's failure to provide onsite washing facilities materially contributed to his dermatitis (Bonnington Castings Ltd v Wardlaw )
House of Lords:D's failure to provide onsite washing facilities was a material contribution to the risk of injury & sufficient to prove factual causation
courts have modified but for test, C must establish D's negligence either: materially contributed to the harm (Bonnington Castings Ltd v Wardlaw ) or materially contributed to the risk of harm (McGhee v National Coal Board )
in Wilsher v Essex Area Health Authority , D only responsible for one of possible risk factors & it could not be shown that this increased risk, so C still unable to satisfy factual causation
issue arises: to what extent is D who either materially contributed to C's harm or risk of C's harm liable for damages?
Holtby v Brigham & Cowan (Hull) Ltd  3 All ER 421
C suffered asbestosis due to exposure to asbestos at work (cumulative condition, which got progressively worse the longer exposure continued)
over several years, C carried out same work for several employers, including D
to what extent was D liable?
C succeeded in demonstrating D's negligence materially contributed to disease, so D responsible for proportion of harm
damages apportioned between D & other employers (the tortfeasors) according to time C worked for each
C must make claim against all tortfeasors to recover full damages
in some cases more than one D has made a material contribution to C's harm but it is not divisible (eg. single injury in a road traffic accident)
Cs had developed mesothelioma (cancer) caused by exposure to asbestos while working for several employers, Ds were some but not all of the employers
medical evidence failed to show which employer responsible for the exposure which led to the cancer, so each D argued but for test was not satisfied
could Ds be held responsible?
Court of Appeal: lack of medical certainty meant factual causation not proved
House of Lords: approved approach in McGhee v National Coal Board , finding Ds materially contributed to risk of Cs contracting cancer & mesothelioma was an indivisible injury & therefore, Ds were jointly & severally liable
another controversial decision followed, appearing to retract scope of Fairchild v Glenhaven Funeral Services Ltd 
Cs contracted mesothelioma working for several employers, D only one still trading
D argued unfair to impose joint & several liability when their breach only contributed to risk of harm & that liability should be proportionate (to extent to which they contributed to risk: time they employed Cs)
could D be held jointly and severally liable?
House of Lords (majority): liability for mesothelioma under Fairchild v Glenhaven Funeral Services Ltd 
, was for risk of harm & therefore D's liability should be proportionate
although mesothelioma was an indivisible injury, the risk was divisible & should be reflected in D's liability
Barker v Corus  heavily criticised for limiting Cs ability to receive damages, so Parliament passed Compensation Act 2006 (effectively reversed decision for Cs suffering mesothelioma)
unclear whether decision will be followed in cases where causation is based on material contribution to risk of other harm
Factual causation: loss of chance
courts must focus on outcome of events not damage which occurred if C argues D's breach caused a loss of chance, rather than the harm
Hotson v East Berkshire Area Health Authority , House of Lords used all or nothing approach but refused to rule out successful loss of chance cases in different circumstances
a few days later, P was descending steep steps without a handrail, he lost control of his leg & fell, severely fracturing his ankle
did P's intervening act break the chain of causation?
chain of causation broken: P took an unreasonable risk
Lord Reid: .. if the injured man acts unreasonably he cannot hold the defender liable for injury caused by his own unreasonable conduct. His unreasonable conduct is novus actus interveniens. T he chain of causation has been broken and what follows must be regarded as caused by his own conduct...
C's carelessness may not always so unreasonable as to break chain of causation but may amount to contributory negligence