The ‘crumble’ is old tires, with the exception of some fields with a white crumble called ‘Nike Grind’ - which is the material used on the soles of shoes.
I suppose the actual inhalation of the crumble could cause a problem.
But nothing in life is risk free.
Looks like ACGIH doesn’t have it under study. Dang, living is dangerous!
http://www.acgih.org/tlv-bei-guidelines/documentation-publications-and-data/under-study-list