Folks,
The vents most likely
are related to a gas appliance, possibly an old one which is no longer present, but you would be wise to investigate carefully before blocking them in any way.
Newer gas boilers generally use a balanced-flue arrangement in which the combustion chamber is completely sealed from the room. Fresh air is drawn from outside, and exhaust gases are vented to the outside, so they do not require any specific ventilation requirements.
Many older boilers, however, as well as most of the old-style oversink geysers did not have sealed combustion chambers, and although they might have an exhaust stack to the outside they need a vent in the room to insure that there will always be a source of fresh air. Without such a vent, they can't always create the necessary airflow to draw the exhaust gases out of the flue.
It's quite common to find places in which the old gas boiler/geyser has long gone, replaced with a modern sealed appliance, but the vents are still there. In this case it is perfectly safe to block the old vents, but
please have your appliances checked first to make sure they are the right type. Without proper ventilation, some open combustion chambers will not operate efficiently, and you'll see a build-up of carbon
monoxide (CO, not CO2), which is an odorless killer.
And yes, sometimes vents were fitted in kitchens simply to help alleviate damp and condensation problems. These were often the type which have shutters operated by a knob so that they can be closed at will. Obviously vents essential for gas appliances should
not be of that type, but it's not unknown for one to have been replaced with the wrong type.
http://www.britishgassafety.co.uk/SafetyIssueFaqs.aspx?cid=13&ctid=3&pid=21
