The harness, by holding the shell away from the skull and absorbing energy as it deforms.

The shell spreads the impact and resists penetration.

The gap between shell and head is the protection rather than slack.

Nothing should be worn or stored between the harness and the head.

A cap underneath, or an incorrectly adjusted harness, removes most of the function.

Adjusting the harness properly takes seconds and is routinely skipped.

Most are held on by the harness alone, so bending forward or looking up sharply dislodges them.

A helmet on the ground protects nobody.

It also becomes a falling object hazard for anyone below.

Chinstraps are available on most models and are increasingly specified.

For work at height a chinstrap is normally required rather than optional.

Climbing-style helmets address retention by design rather than by accessory.

Ultraviolet embrittles the plastic over time, and the damage is not obvious.

A hat left on a dashboard or worn constantly outdoors ages faster than its manufacture date suggests.

A chalky surface or visible crazing indicates a shell that should already have been replaced.

Manufacturers state a service life from manufacture and from first use.

Storage out of sunlight extends life meaningfully.

The manufacture date is moulded into the shell and is what the service life runs from.

After any significant impact, at the end of its stated service life, and whenever the shell shows degradation.

Energy-absorbing deformation may not be visible after an impact.

A hat that has been struck is replaced regardless of appearance.

The harness has its own replacement interval, usually shorter than the shell's.

Crazing, chalkiness, cracks and discolouration are all replacement triggers.

Replacement is inexpensive relative to what it protects.

Only what the manufacturer permits - adhesives and solvents can attack the shell and weaken it invisibly.

Paints, solvents and some adhesives are the specific concerns.

Damage from them is not visible and reduces impact performance.

Manufacturers state what may be applied and where.

Stickers can also hide cracks that inspection would otherwise find.

Site marking should follow the manufacturer's guidance rather than whatever tape is to hand.

Cap-mounted earmuffs, visors, lamps and chinstraps all mount to it, and tested combinations are what the manufacturer publishes.

Slot dimensions and geometry vary between manufacturers.

A muff that physically clips into another brand's helmet has not been tested in that combination.

Accessories can also affect the helmet's own performance if they alter the shell or harness.

Many helmets have a limited number of accessory slots, so combinations compete.

Buying helmet and accessories as a matched set removes the question.

Yes - electrical insulation, lateral impact and temperature ratings all vary, and the schemes differ by market.

Electrically insulating helmets are rated for defined voltages.

Some standards address side impact as well as impact from above.

High and low temperature performance is separately specified.

Marking on the shell states what the helmet is certified for.

Certification from one market is not automatically acceptable in another.