From watching a lot of the public enquiry into Grenfell, a huge number of firefighters who were there never entered the building nor were actively engaged in the fire fighting. When you actually delve into the figures, probably only a few dozen entered the building during the height of the fire and they were wearing breathing apparatus. The vast majority of those attending were in supporting roles or were just there in readiness in case they were needed. You can compare pictures online - the difference in "groups" is clear, there are groups of FF who've clearly been in the building as their uniforms are grimy etc., but by far the bigger groups are those who didn't get used, their uniforms, oxygen cylinders etc are clean! (If the latter group has higher incidence of cancer, perhaps they'd been told to wait too close to the fire!).
So, the figures need breaking down into what each of those 20 FF actually did at Grenfell. DId they all enter the building trying to rescue people? Were they all working very close, i.e. manning ground gets or aerial ladders? Were they just breathing in the smoky atmosphere by simply being close to the fire ( and if so, did a proportion of police, council workers, people living locally, paramedics, etc who were also in close proximity also get cancer due to breathing in the same contaminated air). Was it the FF who'd been in the building and were "dirty" with contaminates, who perhaps should have been told to return to home stations, remove their dirty uniforms and wash/shower, rather than be hanging around continuing to breath in contaminates from their dirty uniforms? Of FF who weren't actually used in active fire fighting, but were waiting around for too long, too close to the fire, breathing in contaminates without their Breating apparatus on.
I hope that some proper analysis of the figures takes place, including other people, (workers and residents) who were close the fire at the time, exactly what the affected FF did whilst at the fire, everyone's history of other potential exposure to carcinogenics, etc.