The amount of tobacco smoke in a room is going to be highly dependent on the box model parameters: source rate, residence time, volume. In relatively large rooms, like a large restaurant, there may be enough air exchange between the inside and outside (from people going in and out) that sidestream smoke concentrations cannot get very high (that does not reduce the annoyance factor for people near a smoker).

"Very smoky" air contains enough smoke to make the air look hazy.

The eye/lung irritation and asthma is from the lachrymators in the tobacco smoke. These include NOx, formaldehyde, acrolein. Repeated and excessive exposure may eventually lead to chronic bronchitis and pulmonary emphysema.

Heart disease/increased risk of heart attack comes from the carbon monoxide (CO). The carbon monoxide reduces the oxygen content of the blood, which leads to a weakening of the heart muscle.

The increased risk of developing lung cancer comes from the carcinogens in the tobacco smoke. These include formaldehyde, nitrosamines, the heavy metals, radioisotopes, and various organic compounds.