Was Nelson's Queen Anne's Mansion designed against gas?

Was Nelson's enclosed Queen Anne's Mansion bridge designed to be gas-proof or kept under positive pressure?

To an extent: it was designed to resist gas by contemporary standards, and the broad idea of pressure-protected warship spaces was not new.

Queen Anne's Mansion was designed, as far as contemporary knowledge allowed, to resist gas entering the command spaces. This offered protection to an extent rather than an absolute guarantee. A comparison with a later overpressured combat space also shows that the underlying idea was not new. The exchange links the bridge's enclosed form to environmental and chemical-warfare concerns as well as command accommodation.