Use scientific methods to measure air flow and quality (CO2), and humidity. You want around 50% humidity and CO2 to be below 700 ppm. CO2 doesn't directly measure air quality, only how often that air is changed. The more like outside (around 300 ppm) the better. 35% or lower humidity will effect your your skin and mucous membrane. A dried out mucous membrane makes it easier to catch a cold. And dried out skin leads to cracks and itching. But too high humidity 80% + is also bad. If the temperature between outside and inside differs more then 7 degrees C, it's enough to effect the humidity, for example if it's cold outside, the cold air expand, leading to less relative water. You can get a humidifier or dehumidifier to control humidity, and use ventilation to control air ventilation. Then there's also temperature, you want around 20-23 C to be comfortable, but I guess you already figured that one =)