Most pre-treatment of drinking water is a physical process involving flocculation, filtration and ion exchange and it is only at the treatment/disinfection stage that chemicals are used, with the resulting gas hazards. Chlorine is the traditional disinfectant used in water treatment - in most countries it is a legal requirement, even if other non-chemical methods (such as UV disinfection) are used While chlorination is still the treatment method of choice in most water works, some are now switching to alternative methods such as ozone, chlorine dioxide or sodium hypochlorite.