Asked by: FOCUSIt would be impossible for any crowd dispersal weapon to be completely safe. Rubber bullets are designed to be fired at the torso, but can be lethal if they accidentally hit the head. Tasers don't break the skin, but critics say they can be harmful to people with heart conditions. Water cannons can tackle a large crowd in one go, but people can be injured by being knocked down or crushed. However, the US military has been prototyping technology called the Active Denial System (ADS). It emits high-energy millimetre waves. These have a wavelength between microwave and X-ray and cause fleeting, localised heating of the skin. The recipient feels an extreme burning sensation that's so acute that volunteers in trials have only managed to withstand it for five seconds, at the most, before running off to escape it. Over 600 volunteers have taken part in tests. There were no long-lasting injuries apart from some blistering in a small number of participants and two cases of second-degree burns.