In the former he found that the bronchial epithelium contained more goblet cells, shorter cilia and a thicker epithelium.

Cells in this membrane, called goblet cells, release mucus to help prevent microorganisms and debris from entering the lungs. The trachea is also lined with tiny hair-like structures called cilia.

Cilia are small hairs which beat to push the mucus back up the trachea so it can be swallowed and destroyed in the stomach. Clean air then enters the two bronchi, one bronchus going to each lung.

A team of scientists created them using human cells from the trachea. Part of the reason why they used those cells is because they are covered with cilia, or tiny, hair-like projections.