The presence of lung surfactant at the air-water interface of the lung is vital to prevent death from respiratory failure. Ozone, a pollutant in ambient air, is known to damage some components of lung surfactant and is linked with death from respiratory failure but the underlying mechanisms of the damage involved are poorly understood. In this proposal we wish to use neutron reflectivity on Figaro to follow changes to the structure of mixed lipid monolayers containing cholesterol when exposed to low levels of gas phase ozone. The lipid mixtures include a saturated lung lipid, DPPC, which is unreactive towards ozone, and an unsaturated lung lipid, POPC, which undergoes significant reaction and rearrangement when exposed to ozone. The results will complement our existing data from X-ray reflectivity experiments on mixed monolayers, which revealed overall changes to the monolayer, but not the changes to the individual lipid components, something available only with neutron methods and selective deuteration. Changes to cholesterol are particularly interesting as it is know to significantly influence the packing and arrangement of lipids.