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The Invisible Killer.

What is the world's most polluted city? Ask this question in the 1950s and early 1960s and the answer would have been London, having attained infamy for the deaths of thousands of its residents during thick pea souper smogs. The title was then passed to Los Angeles with its traffic fumes and emissions from the petrochemical industry. In 2012, the US embassy in Beijing tweeted that the air pollution was 'crazy bad' and the international yardstick for polluted places passed to China. Today we have a new contender for the dubious title.

Over recent years Delhi's post-monsoon air pollution has leapt into the limelight. Each October and November the city is shrouded in a smoky haze that seems to have no end. Like all developing countries, Delhi wrestles with traffic pollution; home burning and cooking smoke; industry and rubbish burning: but what makes the post-monsoon season so polluted? Some fingers have pointed to Diwali fireworks. Diwali was traditionally celebrated with the lighting of ghee-burning lamps, but this changed with the opening of India's first firework factory in 1940. Across India, Diwali fireworks have been linked to a 30 to 40 per cent increase in recorded breathing problems and pollution, but even so we cannot blame them for weeks of oppressive pollution. The cause, perhaps surprisingly, is found not in the city but in the countryside.

Agriculture has changed in India. Increased mechanisation has transformed rice harvesting. Rather than harvesting by hand, rice is now collected by machines, but these leave a lot more roots in the ground. These roots are difficult to clear and so farmers resort to burning the rice stubble. The timing of crop planting and harvesting has changed to improve soil conditions. This change pushes agricultural burning into a time of year when the weather is especially adverse for air pollution. The crop fires are clearly visible on satellite images, as is the smoke that envelops the whole region. Until farmers can be helped to find another solution, Delhi will continue to be plagued by smog each October and November.

It's not just Delhi that suffers from air pollution from agricultural fires. The burning used to clear peat and rainforests in Southeast Asia can spread particle pollution over a wide region. It has been estimated that the air pollution from landscape fires leads to the early deaths of over half a million people globally during the worst El Nino years. Throughout 2018 the news has been filled with images of wild fires across the northern hemisphere. Smoke has caused air pollution problems in nearby cities, exposing millions of people. Moorland fires affected air pollution in Manchester. Cities on the west coast of the US and Canada had to issue pollution alerts as forest fire smoke turned day into night. During a national league soccer game in Oregon players had breaks every 15 minutes and oxygen was provided for them. Smoke can spread across incredible distances: in July, Siberian forest fire smoke crossed the Arctic to reach North America. Smoke from North American forest fires traversed the Atlantic and reached Europe in mid-August, causing purple skies in southern Ireland.

Landscape fires are not the only way that agriculture affects the air that we breathe. Fertilisers and manure used in farming release huge quantities of ammonia into the air. Surprisingly this means that it's Spring in western Europe, the season we associate with renewal, green shoots and flowers, that suffers extremes of particle pollution. In March 2014, Paris had to resort to banning half of the traffic from its roads to control this seasonal menace and it affected London too. The cause, as in Delhi, is not found solely in the city, but in the countryside too.

Spring is the time when fertilisers are applied to new crops and manure that was stored over winter is spread on the fields. This releases ammonia that mixes with the pollution from diesel traffic, gas burning and industry to create tiny pollution particles in the air. These are not black like soot. Instead they cause the haze that obscures the horizon or distant hills. The particle pollution can then stay in the air for a week to ten days, spreading over the whole of western Europe. Fertiliser is expensive for farmers and ammonia blowing away on the wind is wasted money. Agricultural science is helping farmers to use the right fertiliser at the right time, and new technologies can inject chemicals into the soil rather than spraying them, but farmers need to adapt.

Globally, over four million people die early each year due to air pollution. The World Health Organization has labelled this a public health crisis. Governments tend to take action on one air pollutant source at a time. The current focus on diesel cars is an example. My new book, The Invisible Killer, makes the case for managing all sources of air pollution and to stop using the air that we breathe as a waste disposal route. The solution to our cities' air pollution does not just rely on cleaning up traffic or heating. As Delhi, London and Paris illustrate, farmers have a role to play too.

Gary Fuller is an air pollution scientist at King's College London. His new book, The Invisible Killer: The Rising Global Threat of Air Pollution - and How We Can Fight Back, is published by Melville House
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Title Annotation:PLATFORM; air pollution and agriculture
Author:Fuller, Gary
Date:Jan 1, 2019
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