Final answer:
Particles less than 1 micrometer typically affect the alveoli, which are the deepest part of the respiratory system where gas exchange takes place.
Step-by-step explanation:
The part of the respiratory system affected by particles less than 1 micrometer (mcm) in size is the alveoli. Particles this small are able to bypass the natural filtering system of the nasal cavity, which uses mucus and hairs to trap larger particles, and can penetrate deeply into the lungs. The alveoli are where gas exchange occurs, and they are susceptible to damage or inflammation from these very fine particles, leading to respiratory issues like asthma, bronchitis, and pneumonia. In comparison, larger airways like the trachea, bronchi, and bronchioles are equipped to handle larger particles thanks to the protective mechanisms of the respiratory system.