According to OSHA, combustible material can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, under certain conditions, it can become explosive. Even materials that do not burn in larger pieces, given the proper conditions, can explode in dust form.
There is no OSHA standard on combustible dust, but under the Combustible Dust National Emphasis Program, OSHA can issue fines for violations under 18 different standards, including 1910.22, the main umbrella standard. You can also be cited under the General Duty Clause, which can apply to recognized hazards not specifically addressed in OSHA standards.