Can occupational exposures be prevented?

Yes. Look at osha.Org for full listings of prevention for various occupations and substances.
Yes. If you work in an environment with significant risk of occupational exposures, your company should have a safety plan and likely a safety officer. They will be a valuable source of information to help prevent exposures in the workplace.
Usually. Employers have an obligation to protect employees from hazards. There is a stepwise approach to this. First engineering controls are used to remove hazards. If that can't stop all exposure then administrative controls to keep people minimally exposed. If there still is a hazard then personal protective equipment is used.