Some of the lung bacteria protect the lung against bad germs. Tests have shown that people with sick lungs often do not have these ‘good’ bacteria in their lungs. Our team has shown that breathing air pollution causes the lungs to be irritated, leading to inflammation. Inflammation of the lungs can make it hard to breathe, especially for people who have lung problems. We think that polluted air could change the lungs’ bacteria. To understand this, we asked healthy volunteers and people with lung sickness called chronic obstructive pulmonary disease (COPD) to participate in a study. They sat in a room for 2 hours and breathed either clean air or air with diesel exhaust that is a model of air pollution like that found in big cities around the world (i.e. Mexico City, or New Delhi). A doctor checked them and took small samples of fluid from within their lungs. We will use these samples to examine the lung bacteria and evaluate the effect of these bacteria on lungs’ health. This study will help uncover how air pollution affects our lungs and bacteria that are inside the lung. This study will help doctors and governments make decisions about traffic-related air pollution and respiratory health.