An estimated 4,267 Londoners are dying prematurely each year because of long-term exposure to airborne pollution. Those living in inner-city areas breathe in the most polluted air, according to a detailed breakdown of pollution levels in wards across the capital, published by the London mayor, Boris Johnson. The study of the health impact of high levels of fine particulate matter of a concentration level known as PM2.5 on mortality rates in the capital prompted calls for Johnson to introduce 'urgent targeted measures' to address the city's poor air quality, which is the worst in the UK and among the worst in European cities. Airborne pollution in the form of fine particulate matter emanates mostly from combustion sources, including transport, domestic and industrial sources, and aggravates respiratory and cardiovascular conditions. PM2.5 refers to particles of less than 2.5 micrometres. Research shows these particles are likely to be inhaled deep into the respiratory tract and have a disproportionate impact on children due to their smaller lung capacity. Poor air quality is considered to be one of the biggest public health issues now facing the UK. A report by the House of Commons environmental audit committee included evidence that showed pollution could be contributing to 50,000 deaths in the UK each year. Last month, the Government received a second and final written warning from Europe to clean up London's air or be taken to the European court of justice and face fines of up to �300m for being in breach of air quality standards.