From 1 May 2014, all pilots in Cat. 1 events must wear at all times while flying a helmet certified to either EN 966 (airsports), ASTM 2040 or SNELL rs98 (snow sports). That was decided to encourage a broader choice of models and sizes at lower costs, and make more available the protection by helmets fitted well and securely, and replaced regularly.
Moreover, recent research has shown that a significant proportion of the brain damage from impacts is from rotational forces. Technologies such as MIPS are available in snow sports. Before that, wearing EN 966 certified helmets has been mandatory in Cat. 1 events since 1 January 2010.