Already established health care findings suggest using without venting externally means there will be more for you to clean in the long term,the steam and moisture not to mention the grease content after using will eventaully line the walls of the hood and filter system until the room in finally saturated with a hard to move residual film covering all the immediate areas surrounding your cooking area,this will cause a build up of unhealthy germs in this enviroment and is bad for the people using this area,

External venting will reduce the risk of damage to the buildings roof and celing structure,cost of venting versus cost of cleaning and or repairing rot damaged ceiling cavitity is a great way to weigh up your healthy options,

Sometimes rangemaster hoods have a filter system. check with them if your model does this. nuber for tech help by them on internet.