Smokers must be greater than 20 feet from a public building's entrances or operable windows while in the act of smoking. This is governed by California Government Code, 7597. This doesn't apply to portions of the building that don’t have operable windows, doors, or the like. You should also consider posting signs near any HVAC air intake vents.

7597. (a) No public employee or member of the public shall smoke any tobacco product inside a public building, or in an outdoor area within 20 feet of a main exit, entrance, or operable window of a public building, or in a passenger vehicle, as defined by Section 465 of the Vehicle Code, owned by the state.