Assuming you mean halon... There are 2 issues: the manner in which they work... Sucking all the oxygen out of an enclosed space thus killing the fire and operatives trapped inside; as a case in point I conducted a HS review on the Thames barrier.... Under the water are maintenance tunnels involving lonesome for issues, and arguably CSE issues too. These tunnels where halon 'protected' naturally one of my main recomdndations was to displace halon Asa priority. Secondly, there's an environmental issue: halon is significantly more efficacious in terms of greenhouse gases than the benchmark of carbon monoxide. These tables are easily found on the net. I hope this helps