Chloroform is slowly oxidised by Oxygen present in air in the presence of light to Carbonyl Chloride (Phosgene) - an extremely poisonous gas - by the following reaction,

\(2CHC{l_3} + {O_2}\xrightarrow{{light}}\mathop {2COC{l_2}}\limits_{Phosgene} + 2HCl\)

By storing it in closed dark coloured bottles, this is prevented.