An employer must not require or permit an employee to work in an environment in which the time-weighted average dry-bulb temperature, taken over a period of four hours is less than 6 ºC, unless the employer takes reasonable measures to protect such employee against cold and further takes all precautions necessary for the safety of such employee: Provided that, where outdoor work is performed, the employer shall take such measures and such precautions in an environment in which the actual dry-bulb temperature is less than 6 ºC at any time.