Is there a link from working with steam boilers to leukemia (cancer)?

can and or is there any body out there that has or knows any information if theres a link to being a steam boiler eng.(fire tube) powered by natural gas and contracting leukemia. see here it is my father age 56 resently past away from leukemia and being a steam boiler engineer for more than 40 years also 5 others have past within the resent years and all family guys that i grew up around.and there job to maintain and servise the boiler room containing 3 boilers to service a hospital.would there be a link in having to service the boilers inner linnings and fire bricks and tubes for that matter?any information,data,fats,info for that matter would be gratefully loved. nobody of any of these family have had any real closure to there ping.but im working on it. so i thank all up front for any information that comes my way.thank you rudy (age38) R.I.P. i got you pops.