Occupational Safety and Health Administration has published safety tips for working in the cold winter weather.

From OSHA.gov:
Outdoor work requires proper preparation, especially in severe winter weather conditions. Although OSHA does not have a specific standard that covers working in cold environments, employers have a responsibility to provide workers with employment and a place of employment which are free from recognized hazards, including winter weather related hazards, which are causing or are likely to cause death or serious physical harm to them.