Airport pollution in some developing countries, such as China and India, can be very heavy. The polluted air contains particulate matter (PM) in different size. The PM could have an adverse impact on the performance of environmental control systems (ECS) of airplanes that frequently serve in those countries. This study is to investigate how much and where the PM is deposited in an ECS through ground experiment and in-flight tests. Numerical simulations of the particle deposition are also conducted and compared with the experimental data.