In most large cities air quality is poor. Fine particulate matter (PM), exhaust fumes and aerosols contaminate the air that we breeth. According to new results, toxicity of PM matter might have been underestimated due to methodological shortcommings. For analysis with an offline method samples are collected (e.g. on a filter) over several hours and later extracted and analysed in the lab. Reactive organic compounds with a short half-life degrade during the delay. Using an online method samples are analysed within minutes of collection. Therefore, reactive components are still present and detectable. Researchers from the University of Basel compared online with offline methods on actual air samples from cities like Beijing and Los Angeles. They also measured cytotoxicity of fresh and aged PM samples on human bronchial cells. Oxidative potential and cytotoxicity of fresh samples was much higher, compared to samples that had aged for several hours.