“But, the compliance of a particular helmet with the designated ASTM standard is predicated on the assumption that one will wear the helmet under the same conditions in which it passed the ASTM testing.”
I’ve learned ASTM testing involves steel headforms. Hair doesn’t seem to factor into it all. Here is a link to what the forms look like – one would presumably have to be completely and utterly bald to fully replicate the simulation.
This does make the point that a closely-fitted helmet is optimal but how one gets there will vary – thus, as has been pointed out, the most important thing you can do is be fitted with the help of a professional.
The Charles Owen site overviews testing and shows the headforms in use, theirs adds computer simulations – there’s also a great Laura Kraut video about her personal experience.
The only study I can find related to hair and helmets is one for football helmets, funded in part by the NFL. PDF here. While football players don’t wear their hair up, this report does note “The fit of football helmets varies widely [1] and football players have varying amounts of hair, ranging from bald to long dreadlocks” – therefore, even down, fit is still an issue and not necessarily guaranteed.
The American Academy of Orthopaedic Surgeons recommended for high-school football players in 2016 that “The risk factor of poor helmet fit should be minimized through mandated adult supervision and midseason spot checks” because “Helmet fit can change and evolve during the season and games, due to sweat, hair style, rain, cold weather clothing, and other factors.”
Presumably spot-checks are more hands-on than visually observing the athlete’s hairstyle.
As a side note, I was amused to see the NFL has the opposite problem - players’ hair, flowing free from their helmets, has been a point of discussion and proposed rules given the dangers associated with hair-grabbing and pulling during tackles.