What is staggering is that the airlines themselves will be sitting on data.
A well structured study could easily show a statistically significant correlation between the health impact of continued exposure to toxic air by looking at sickness rates in the flying crew population and the broader population. There is research into the likely presence of carcinogens and the diagnosis rates for flight and cabin crew. If this rate is statistically different to the rate in the broader population then it is possible to establish a prima facie link between the repeated exposure to the air and illness (cancers in particular)
What is suprising is that a union as a 'welfare' or health and safety investigation do not launch a structured plaintiff driven case to discover this data (held by airlines).
Given a balance of probability standard is a lower threshold, it is likely any well constructed law suit would never get to the court steps...