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You should choose your ski helmet based on 4 criteria: size, ventilation, lightness and visor
To get your helmet size, use a tape measure to measure your head size, passing well above the ears, eyebrows at the front and the widest part of the skull at the back (on the occiput). The objective is to measure the circumference of the skull at its largest point.
Between 2 and 8 ventilation holes