The flash momentarily activates all photoreceptor cells in the eye, blinding it for approximately five seconds. Afterward, the victim perceives an afterimage that impairs their aim. The sheer volume of the detonation also causes temporary deafness in the victim and also disturbs the fluid in the ear, causing a loss of balance. Despite the nonlethal intentions behind the grenade, the resulting concussive blast still has the ability to cause injuries, and the heat created has been known to ignite flammable materials. The fires that occurred during the Iranian Embassy siege in London were caused by stun grenades coming into contact with flammable objects.
Stun grenades are usually designed to be non-lethal and cause no damage to the target. However, permanent hearing loss has been reported. Also, other injuries and deaths have been officially attributed to their use. These include the following:
List of incidents
In 1989, police in Minneapolis, Minnesota, conducted a drug raid at the home of an elderly couple, Lloyd Smalley and Lillian Weiss, after receiving inaccurate information from an informant. The stun grenades police used in the raid set the home on fire. Police said they were certain no one was inside, and so, at first, made no attempt at rescue. Smalley and Weiss died of smoke inhalation.
In May 2003, a woman named Alberta Spruill died from a heart attack after a police team detonated a stun grenade at her residence in Harlem, New York while looking for a drug dealer who was already in police custody. Her family eventually won a $1.6 million civil suit against the city.
In February 2010, police in Minneapolis, Minnesota, raided the apartment of Rickia Russell looking for drugs. While Russell was eating dinner with her boyfriend, the police threw a stun grenade after breaching the door. The exploding stun grenade gave Rickia 3rd degree burns on both calves and burns to her head. No drugs were found in Russell's apartment and the Minneapolis City Council agreed to pay $1 million in damages.
In January 2011, a California man named Rogelio Serrato died of smoke inhalation after a stun grenade launched by a police SWAT team ignited a fire in his home. The man was believed to have been hiding in the attic when the fire broke out.
In February 2011, a North Carolina SWAT police officer was injured at his home when a stun grenade accidentally detonated while he was attempting to secure his equipment. He underwent emergency surgery, but later died of his injuries.
On 28 May 2014, a 19-month-old baby boy's face was severely burned and mutilated when a stun grenade was thrown into his playpen by a SWAT team serving an arrest warrant for a suspected drug dealer in Cornelia, Georgia. The baby survived with facial disfigurement. The family received $3.6 million in settlements in 2016.
On 3 August 2014, a Macedonian fan of the FK Vardar football team was seriously injured during a football match after trying to throw a stun grenade used by police when a fight between the police and the fans broke out in the stands of Stadion Tumbe Kafe stadium in Bitola. The grenade exploded in his hand causing him to lose two fingers and suffer severe damage to the structure of his arm.
In August 2018, Michelle Fawcett suffered severe chemical burns and other injuries when a stun grenade was fired into a crowd of antifascist protesters in Portland, Oregon and struck her on the arm.
In October 2019, a 19-year-old undergraduate student suffered a severe head injury when a stun grenade was fired into a crowd of protesters outside the Greek Parliament in Athens, Greece by police. 
On 14 November 2019, a protestor in Baghdad died in a hospital from wounds resulting from a stun grenade that was thrown into a crowd of protesters during a clash with Iraqi authorities.