What size particle can reach the lowest parts of the lung?

Particles that can reach the lowest parts of the lung, specifically the alveoli where gas exchange occurs, are typically very small in size. Particles that are 2.5 microns or smaller are generally able to bypass the upper respiratory system and reach the lower lungs effectively. Among the options provided, particles that are 3 microns in diameter are still small enough to penetrate deep into the respiratory system, reaching the alveolar region.

Larger particles, such as those that are 5 microns or more, are more likely to be trapped in the upper respiratory tract due to the body's defense mechanisms, such as mucous and cilia, which filter and trap larger particles before they can enter the lower lungs. Therefore, the ability of smaller particles to reach deep into the lungs underscores why the choice of 3 microns is particularly relevant in discussions about airborne pollutants and respiratory health.