Passive monitoring doesn't involve pumped air. It has a reactive surface exposed to air onto which pollutants are collected either by physical or chemical adsorption. The passive monitor is usually small and unobtrusive (e.g., a plastic test tube) that is typically left outside for a month before being sent to a laboratory for chemical analysis and calculation of the pollutant concentration. The pollutants typically monitored by this method in New Zealand are benzene, nitrogen dioxide and sulphur dioxide. While this method cannot be used for compliance monitoring against the NES-AQ, it can be used to collect air quality measurements using a network of many samplers over a large area.