Pollutant emissions caused by nitrogen dioxides released by diesel vehicles are a direct cause of contamination in cities worldwide and negatively affect human health, causing the death of 800,000 people a year.

To halt the rise in levels of polluting particles in dry seasons (wet weather cleans the atmosphere), many local authorities impose sustainable mobility actions and restrictions on on private vehicle traffic, which is the ugliest when it comes to emission levels per passenger. Here are some examples.

In a city that, in 2016, reached 192 on the metropolitan Air Quality Index (0-50 is low, 51-100 moderate, 101-150 high and 151-200 very high), restrictions on cars have been extended to weekends.

The Don’t Drive Today program begun in 1989 limits traffic via an emission-based color and hologram system for each car, allowing it to be driven a certain number of days per month depending on its classification. Hybrid and electric vehicles are exempt.

At the end of 2016, Beijing activated the red alert for high levels of atmospheric pollution. It had reached 420 micrograms of PM2.5 particles (the most damaging to health), well over the danger threshold of 300 set by the world Health Organization.

In 2008, because of the Olympic Games, the Chinese capital obliged its inhabitants to use private vehicles a maximum of one working day per week and travel by public transport the rest of the time.

Success was immediate - a 30% reduction in polluting particles – and the city continued applying the measures for some time after the Games had finished.

Health experts recommended in January 2017 that Londoners with respiratory problems limit the number of hours they spend outside, after City Hall declared a red alert for pollution in seven districts of the capital.

London has applied a congestion charge since 2003 which seeks to deter people driving in the city center. Private vehicle users who want to drive in the area within the central ring road must pay a fee that can be reduced with advanced notice of the journey.

Madrid has not complied with EU air quality standards since 2010 and was only able to get below the 40 microgram/m³ NO₂ limit in one year, 2014. Pollution levels fell considerably when the Spanish capital decided to prohibit traffic for the first time.

The last major European capital to introduce traffic abatement measures in its city center, in 2016 Madrid established a three-pronged approach: speed restrictions; parking bans, and; prohibition of access to vehicles with odd or even license plate numbers on alternate days.

Paris reached its highest levels of atmospheric pollution in a decade in 2016, forcing it to introduce an environmental strategy including free transport, diesel vehicle restrictions and the construction of a light railway crossing the city alongside the Seine river.

Since 2014, the city has been restricting traffic on the worst days of particle pollution as a function of license plate numbers, although it first experimented with this solution for one day in 1997. It applies the measure in what is known as the small ring ‘(la petite couronne’), formed by the capital plus 22 surrounding municipalities.

The city reached 180 micrograms of PM2.5 in 2016, according to the Bogota Environmental Observatory, well above the 100 considered damaging to health.

Bogota introduced its “Peak and Plate” program for the first time in 1998. It has since been exported to other cities in Colombia, Ecuador and Venezuela. The measure allows vehicles to pass through certain roads in the city as a function of number plate and the hours during the day when there is greatest congestion. Public transport in the city is not an efficient or safe alternative to car use, however, says mobility expert Fernando Rojas.

In a country where 1.1 million people die every year due to atmospheric pollution, the capital has a concentration of 473 PM2.5 when 99 is the maximum considered acceptable by the World Health Organization.

New Delhi became the city with the worst atmospheric pollution in the world in 2014, says the WHO. In 2015, it thus launched a plan to restrict drivers to alternate times when they can enter the city, according to odd or even license plate numbers. Access to heavy lorries is also prohibited during the day; they can only enter the city after 11pm.

Restrictions have been introduced reducing traffic by 20% (about 300,000 cars) in winter, when the city reaches a state of pre-alert, 300-499 micrograms of noxious particles/m³.

The Chilean capital began applying traffic restrictions according to license plate in 1986, but then they were only applied to vehicles without a catalytic converter. As more and more vehicles had a converter fitted, the city went on to apply the measure to all cars without distinction in 2008.

In 2016, the pre-alert level was breached over a dozen times, and on one occasion the alert level was reached when pollution rose above 500 micrograms.

A report by the University of Athens in the year 2000 was already warning about pollution in the Greek capital reaching levels harmful to human health. The normal level of pollution of between 1-6 mg of CO₂/m³ was on many occasions rising to over 10 mg/m³, the maximum allowed by the Greek authorities. The report says that 3% of heart disease deaths are directly related to bad air quality.

The Greek capital is a pioneer in systems that curb traffic. Back in 1982, it alternately banned odd and even numbered vehicles from crossing the restricted central area – known as Daktylios – from Monday to Friday. Buses, motorcycles, taxis, hired cars and foreign license plate vehicles are exempt from the regulation.

Sources: La Información, El Blog Salmón, Ecologistas en Acción, Living in Greece, Gobierno del Estado Mexicano, Bogotá DC, El País, La Opinión, El Economista, El País II, El Ibérico, El País III, Huffington Post, El Espectador, Univsión, New York Times, CDN, Medicina TV and El Comercio.