268Where the concentration of airborne chemical agents or mixture of chemical agents or airborne dust in a confined space is hazardous to the health or safety of an employee or where the percentage of oxygen in the confined space is less than 19.5% by volume, an employer shall ensure that an employee who enters the confined space uses appropriate respiratory protective equipment capable of providing at least five minutes reserve of unaided life support beyond the time the employee is expected to be in the confined space.