The International Society for Equitation Sciences has issued a position statement on nosebands with recommendations for training and competition, including checks by ring stewards.
The ISES states:
"Recent evidence suggests that horses wearing tight nosebands undergo a physiological stress response, are sensitised to bit pressure and may have reduced blood flow (McGreevy et al., 2012). Consequently, on welfare grounds, the use of nosebands that constrict with potential to cause injuries should not be permitted in training or competition."
"ISES recommends that all equestrian sports should require that the tightness of any noseband is checked by a steward at the nasal midline"
For the complete background and recommendations, go to:
http://www.equitationscience.com/position_statements_nosebands.html