A hard hat is a helmet-like protective head covering used to shield the wearer from painful or damaging blows to the skull. Once made of metal, modern ones are made of sturdy plastic. Besides the hard outer shell, the hard hat also has a fabric mesh or straps on the inside, set about an inch or so away from the shell. This mesh is what rests on the worker's head, instead of the shell, to provide a degree of shock absorption due to especially heavy blows. Additionally, an adjustment mechanism of some kind can be found on the back, which is used to adjust the hard hat to the wearer's head for a secure fit. Some have a brim around the edge to direct liquids away from the wearer, in case of rain or liquids spilled from above. Others have a short bill in the front like a baseball cap.
Hard hats can offer several attachments depending on design. Some have ear protection built in to the side. Some have a spot to attach a face shield for tasks like grinding and pouring chemicals. Others can attach a welding mask. Usually these are removable so they don't get in the way when not needed.
Hard hats come in many colors, signifying nothing. Some companies use different colors to help differentiate workers from each other (mechanics, electricians, outside contractors, foremen, etc), but this is an internal decision and is not standardized. Most hard hats are white. Yellow is also a popular choice due to its high visibility. Some workers will decorate their hard hats with stickers or paint to personalize them. This is fine as long as it doesn't reduce the effectiveness of the protection or interfere with the worker's field of view.
The hard hat is intended to protect people working in mining, industrial, or construction areas. They provide excellent protection against bumping the head on low hanging objects and against tools or material dropped from above. They are not intended to replace other personal protective equipment, such as eye protection, only to supplement them. Unfortunately, hard hats are uncomfortable as they are rigid, do not breathe well, and require a tight fit to do their job properly, so workers do not like to wear them unless absolutely necessary.
When they are necessary, a hard hat could very well save a person's life. Blows to the head are among the most serious injuries that can be sustained on a job site, because although the skull generally offers good protection to the brain, it was never designed to shield it from today's technological marvels. Severe blows to the head could result in shock, unconsciousness, brain damage, coma, or death depending on how hard the impact is and where exactly it hits.
Do not put anything inside the hard hat, such as keeping gloves between the mesh and the shell. This defeats the shock absorption properties intended by the offset between the mesh and the outer shell.
Do not use the hard hat for sports protection, it is not designed to protect the wearer from the sort of injuries which occur in, for example, hockey or cycling. Likewise, sports equipment is not appropriate for construction protection.
Do not continue to wear a hard hat that has been damaged by an impact, discard and replace it.
Do not wear a hard hat for more than five years, they come with a date stamp molded in to the shell and should be discarded and replaced regularly.