tolerance of oxygen deprivation, it is highly vulnerable if vasoconstriction is not reversed. Normally, if the brain is hypoxic, autonomous systems in the body divert blood to the brain at the expense of other organs; because the brain is vasoconstricted this mechanism is not available. Vasoconstriction is only reversed by the build-up of carbon dioxide in the blood through suspension of breathing. In some versions the bear-hug is replaced by pressure on the neck in which case blackout is a hybrid of strangulation and self-induced hypocapnia. Other mechanisms Unconsciousness may be induced by other methods although these are controversial: pressure over the carotid sinus may induce syncope (fainting) without any other action at all but this is difficult to reproduce and is not the basis of the game. For those susceptible to carotid sinus syncope, of which most people would be unaware until it occurred, this can be an exceedingly dangerous game. In both strangulation and self-induced hypocapnia blackouts the victim may experience dreaming or hallucinations, though fleetingly, and regains consciousness with short-term memory loss and involuntary movement of their hands or feet. Full recovery is usually made within seconds if the strangulation stops. Prevalence A 2008 Centre for Addiction and Mental Health study found that at least 79,000 students in the Canadian province of Ontario participated in this act. The 2006 Youth Health Risk Behavioral Survey in Williams County, Ohio found that 11% of youths aged 12–18 years and 19% of youths aged 17–18 reported ever having practiced it. A challenge, named the "Blackout challenge", became widespread on TikTok in 2021, resulting in multiple falaties from children. Injuries and fatalities Any activity that deprives the brain of oxygen has the potential to cause moderate to severe brain cell death leading to permanent loss of neurological function ranging from difficulty in concentration or loss of short-term memory capacity through severe, lifelong mental disability to death. Statistics on fatalities and neurological damage are controversial; no definitive, empirical study exists although the indications are that the practice is a significant contributor to death and disability, particularly among male juveniles in most developed countries. Many believe that deaths are significantly underreported because of false attributions to suicide. One study by the U.S. Centers for Disease Control and Prevention (CDC) found sufficient evidence to indicate that since 1995 at least 82 youths between the age of 6 and 19 have died in the United