I have a semi detached house built around 1950 with cavity walls that have been insulated in the past by a previous owner. Each room has these rather ugly air bricks that I would like to able to brick up and plaster over, I appreciate that the one in the kitchen where the oil boiler is and the one in the front room will need to stay (we use an open fire sometimes in the front room) but what about the others, can these safely be filled in?

The short answer is no.

A free flow of air reduces condensation and thus the potential for mould (aside from the other issues already identified).

One option (often seen in relatively modern houses) is to have trickle vents above the windows. These have the required cross section for airflow and thus the airbrick can be filled in. Normally the only option for trickle vents is with replacement windows... So possibly not the solution you are looking for.

Why not ask the opinion of your local council building inspector. Homes in the 1950's were not double glazed or centrally heated so specifications have changed and hopefully your boiler has a balanced flu and your bathroom has an extractor fan. Any room with a open fire / log burner does need an air supply and more importantly a carbon-monoxide monitor. FL2 MY10 TD4 GS traded in at 2 years
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no air bricks in my house, but as an old farmhouse it has its own leaks so Im not concerned, Also Mrs G opens the windows so many times that Im considered fixing an alarm type loop stopping the boiler firing up if a window is open At work
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pfjths wrote:

The short answer is no.

A free flow of air reduces condensation and thus the potential for mould (aside from the other issues already identified).

One option (often seen in relatively modern houses) is to have trickle vents above the windows. These have the required cross section for airflow and thus the airbrick can be filled in. Normally the only option for trickle vents is with replacement windows... So possibly not the solution you are looking for.

Another reason to keep the ventilation bricks or some other means of ventilation is, depending on where you live and what type of sub-strata your house is built on is Radon gas build up. Radon is a naturally occuring radioactive gas and usually occurs in granite and sand stone. Some parts of Scotland, the south west and the midlands have the highest levels of Radon. Usually normal ventilation is adequate to remove it but in some areas other measures are needed.

J