RANGE HOOD FILTERS: s.sdcpm.com/hood

1:Range hood filters must work correctly and be of the correct type:

2:Plain screen intended for a fully vented hood.

3:Carbon filter removes smoke before venting the air back into the room

4: General advice below re range hoods and filters
The range hood can be a source of grease fires. The range hood serves a few functions in the kitchen. Most of us think that it is meant for ridding the home of cooking smells. Alright, it may accomplish that goal, depending on the type of range hood that you have. The main function of the range hood should be to rid your home of moisture from the cooking process. If you have a recirculating type (a unit that blows the air taken from the cooktop back into the house), you need to open a window to vent the moisture from cooking. The next function of the range hood is to remove grease from the air. Small bits of grease will travel up into the air with the moisture. With the fan on, the range hood takes this air, passing it over a filter, and then out of the unit. The filter catches the grease, but many people forget to clean it. These are wire mesh filters that either slide into place over the fan, or they clip into an opening which leads to the fan.  You can wash the filter like you would your dishes. What happens on the filter is a build-up of grease can drop like moisture that is collecting on a leaf. Once the grease drops, it can hit the hot surface.  I have also seen grease collect on the other surfaces of the range hood, so wiping this appliance down is a good idea. Lastly, a word of advice about a range hood, they often have a light for the cooking surface (I would say always, but you do have different types of range hoods). Make sure that this light is covered. If the light explodes, you will have that glass and other substances in your food. Throw out the food; you do not know what may have dropped into it.

A simple test is to hold your hand up in front of the exhaust port which is usually at the top center on a range hood and see if there is enough force to move the air. A strong flow indicates a clear filter. 
 The range hood filters are very easy to inspect and should be replaced at any sign of grease no matter how long they have been in service.