different when the filters were removed. Particulate concentrations per milliliter ranged from 0.3×10⁹ to 3.3×10⁹, depending on whether the cigarettes were rated ultralow, low, or medium in tax content.

Hinds (1978) compared the particulate size distribution in cigarette smoke using an aerosol centrifuge and a cascade impactor. Although these devices are based on different physical principles, the MMAD values were comparable to those measured by McCusker et al. (1983), ranging from 0.37 to 0.52 µm. Variations depend primarily on the dilution of the smoke. Keith and Derrick (1960) used a specially modified centrifuge, termed a conifuge, to analyze cigarette smoke and reported MMAD and concentration values similar to Hinds (1978) and McCusker et al. (1983). Particulate analysis by a light-scattering photometer yielded a MMAD of 0.29 µm and particulate concentrations of 3×10¹⁰/ml.

Time and concentration can modify tobacco smoke. Cigarette smoke aerosols contain volatile components, and evaporation gradually reduces particle diameters. It is also true that when the particle concentrations are extremely high, like those encountered in mainstream smoke, the aerosol can agglomerate rapidly because nearby particles collide with each other and coalesce. If smoke is cooled (reducing the vapor pressure of volatile components) and diluted in room air (reducing the probability of particle collisions), the size of the particles will become more stable. Particle size may also change within the human respiratory tract. After air containing smoke is drawn into the mouth and upper respiratory tract, it becomes humidified. Smoke particles can grow in size because of their affinity for water, termed hygroscopicity (Hiller, 1982a).

Particle size is a critical factor in determining the collection efficiency, but breathing pattern is also important For example, large slow tidal volumes will favor alveolar deposition, while high inspiratory flows will promote deposition at bifurcations in the airways. Breath-holding is also important. The greater the elapsed time before the next expiration, the higher the fraction of inspired particles deposited, since there is more time for particles to sediment or diffuse. Individual anatomic differences may influence the amount and distribution of deposited particles. The cross section of airways will influence the linear velocity of the inspired air.