Many newer houses and apartments have no air inlets at all. They are literally as tight as drums. Chemical, biological, and particulate pollutants, moisture, carbon dioxide, skin flakes, dust mite feces, soot and cat dander all accumulate in the air of the home. Moisture is one of the worst offenders. It promotes the growth of dust mites, yeasts and molds.
Moisture can even accelerate the out-gassing of some harmful chemicals.
No, leakage is not the same as ventilation. Residents of old leaky houses do get more fresh air, but the air they get is usually neither where, when or in the amounts they need. In other words you have no control over the ventilation in a leaky home. A good ventilation system has two basic jobs. The first is ventilating the whole house, all the time.