Comfort and fit
If a helmet is too big, it can move around on your head and won’t properly protect you in a crash. Too small, and it won’t sit right on your head, compromising your safety and comfort.
Are you looking for added warmth in the cold? Or do you spend more time trekking uphill for backcountry runs? Different helmets have different features, including adjustable vents for added ventilation or detachable liners or ear pads for warmer spring conditions.
Make sure the helmet is designed with a low-friction layer that’s intended to help reduce rotational force to the head during certain angled impacts.