How cities can help protect citizens from air pollution

April 11, 2014

–Julian Hunt is former Director-General at the Met Office and Visiting Professor at Delft University of Technology. Amy Stidworthy is Principal Consultant at Cambridge Environmental Research Consultants. The opinions expressed are their own.–

The Saharan dust in London last week affected the atmosphere, and caused irritation to the many people who suffer from breathing difficulties. Just as in the smog of the 1950s and of Dickens’s day, which was caused by soot from coal burning, the cloud of dust particles was dense enough that less sunlight made it through to ground level.

When this happens, polluting gases and particles of all kinds are not dispersed upwards, and are concentrated in layers below the tops of the tallest buildings. When finally the sun broke through last week, the dust was dispersed and for those suffering from the effect of the dust there was some relief.

But London is fortunate because such meteorological events are very unlikely to occur in the height of summer when it may coincide with a heat wave. Other countries, in southern Europe for example, may not be so fortunate. In large Asian cities, huge sandstorms, dust from coal combustion and industrial sources, and emissions from motor vehicles can lead to health effects so severe that an 8-year old girl living near a crossroads was reported to have died from particle-related lung cancer.

As key global cities collaborate and compete with each other in improving environments, they all want to ensure that they have the most up to date methods for monitoring, forecasting and communicating air pollution, especially during critical events. Beijing and London both tested  methods during their respective Olympic Games in; now as the central areas of London become more like Hong Kong, UK urban modelling is benefitting from collaboration.

Some cities rely on a statistical approach to forecasting and regulating air pollution, for instance in Italy and France – to control traffic entering city centre areas, but their dependence on past data means they cannot predict changes in future trends.

Other cities, including London, also make use of ‘bottom-up’ computer modelling of all the relevant scientific processes and estimates of future trends. There are two processes involved.

First, weather and atmospheric chemistry data on a regional scale are collected and forecasts computed (at the European Medium Range Weather Forecasting Centre in Reading and at the Met Office). These forecasts take account of events such as sand storms, forest fires, volcano emissions and movement of national-scale pollution, making allowances for urbanisation and the effect of changes in climate.

Second, the environment and pollution within and around urban areas is forecast, so as to inform communities, organisations and people about implications for their operations and health. With recent developments in urban environmental modelling (for example the ADMS-Urban modelling system developed by UK research groups), forecasts of pollutants are possible on a street-by-street level, based on meteorology, atmospheric chemistry and emissions.

The highest concentrations are regularly predicted along narrow traffic-laden streets in London, which often feature children, and vulnerable adults in schools, hospitals and social housing. Here, prior warning of high pollution can help; the airTEXT service delivers forecasts for free to Londoners by text, email and voicemail (sign up at or text ‘airtext’ to 78070).

But the experience of last week demonstrated again, as in studies of urban heat-waves including those in Europe in 2003, that forecasts and warnings need to be related more precisely to health impacts. The warnings of bad air pollution, which are based on forecast concentrations of gases and small dust particles (less than 10 and 2.5 microns), do not allow for the great variability in their impact on individuals: some are sensitive to ozone, others to humidity plus particles, and others to daily number of hours of high temperature in a heat wave.

Across an urban area, for example near parks and water areas, or in streets lined with tall buildings, atmospheric effects and their impacts can vary greatly. With more research to define these classes of impacts in urban areas, it will be possible to provide more focused warnings to the people and communities vulnerable to urban air pollution, which regrettably is still worsening year by year in some of the largest cities of the world.

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