The presence of lung surfactant at the air-water interface of the lung is vital to prevent alveolar collapse. Exposure to environmental pollutants can cause the formation of oxidising species within the aqueous phase that potentially damage the biophysical properties of the surfactant layer, with adverse health effects. We wish to study changes to the structure of natural (porcine) lung surfactant at the air-water interface when exposed to the aqueous phase oxidant OH. The results will complement our existing data from showing the damage caused by the gas phase oxidant ozone.