Q – Can my guys wear a winter knit cap while wearing a hard hat?
A – In a Letter of Interpretation dated April 17, 2006, OSHA specifically answered the question of whether the regulations prohibit an employee from wearing a scarf, cap or other item on his head for the purpose of cold weather protection, while wearing a hard hat.
OSHA stated that while the regulations do not specifically prohibit the use of cold weather garments under hard hats. All hard hats must conform to ANSI Z89.1-1969 and Z89.2-1971, which recommend the use of “winter liners” that are specifically designed for use with hard hats. They stressed that the “winter liner” must be compatible with the hard hat’s protective properties and implied that a knit cap would likely interfere with the hard hat’s protective properties.
Finally, OSHA clarified that some manufacturer’s, like Bullard, prohibit the use of any accessory inside a hard hat. Bullard believes that the use of even a liner: “may limit the necessary protective clearance between the hard hat shell and the wearer’s head”. April 17, 2006 OSHA Interpretive Letter to Mark Kasel,https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=25336
However, Both FibreMetal and MSA sell hard hat liners designed to be worn under a hard hat in the winter. Diamond Tool carries a wide variety of safety products including hard hat liners:
Diamond Tool also provides: safety consulting, safety manuals, safety inspections and a full range of OSHA safety training. http://www.diamondtool.net/pg/diamond-safety-services