"We go now, we are going, we go now, please, please... we are leaving in one minute to Athi River," shouts driver Samuel Mburu as he gesticulates to passing pedestrians, trying to entice them to get on his 23-seater bus (or matatu) in the Central Business District of Nairobi, Kenya.
Thirty minutes later, Samuel and his brightly-painted matatuis still sitting with its engine running at the Nairobi Railway Station, one of the main passenger collection points for matatus in Nairobi. The matatu has still not left for the town of Athi River--nearly two hours by bus from the city-- but its engine has been purring the entire wait time.
Engine idling is the act of leaving a vehicle's engine running while it is stationary. "If we leave the engine running, it looks like we are about to leave which attracts more customers and people don't want to get on a bus when the engine is not running", says Antony who works as a bus conductor. "We leave the engines running as the engine might not start again if I turn it off," adds Samuel.
In those 30 minutes since Samuel's matatu was supposed to depart, harmful pollution from the exhaust of Samuel's matatu is released into the air.
All over Nairobi, one of the world's most gridlocked cities, thousands of matatus and other vehicles idle their engines, sending millions of small harmful, polluting particles into the city's air.
Health effects of engine idling
As the World Health Organization explains, particles with a diameter of 10 microns or less, can penetrate and lodge deep inside the lungs.
The even more health-damaging particles are those with a diameter of 2.5 microns or less. Particulate Matter 2.5 includes a variety of components such as nitrates, sulphates, organic chemicals, metals, dust and black carbon. Due to their small size, they penetrate the lungs and are known to cause heart, lung and other diseases. The World Health Organization estimates that nearly 19,000 people die prematurely in Kenya annually because of air pollution.
One of the causes of these deaths are PM 2.5 annual exposure, which, according to the UN Environment's Breathe Life campaign are 70 per cent over the safe level in Nairobi.
Indeed, matatus with idling engines near schools are particularly harmful as children are more vulnerable to the effects of air pollution. One reason why children are particularly vulnerable to the effects of air pollution is that they breathe more rapidly than adults and so absorb more pollutants at a time when their brains and bodies are still developing. Children also live closer to the ground, where some pollutants reach peak concentrations.
"I have lung problems and trouble with breathing. I even cough up black dust because I spend so much time here," says Dennis Ngungu who owns a company which runs 70 buses in Nairobi. He stands beside one of his idling matatus about to leave for Banana Hill in Nairobi.
Two friends, Roise and Jane are tightly squeezed into a matatu waiting to depart for Machakos County, located approximately two hours outside Nairobi. "The smoke from the matatus really disturbs your lungs and I prefer when they switch their engines off when they are stationary," says Roise. "If I see a matatu with the engine off, I don't want to get in it as it doesn't look like it is leaving," adds Jane.
At the same bus stop, exhaust fumes from a departing matatu billow directly into the face of three-year-old girl of, dressed in a fluffy pink ski jacket.
Can the practice of engine idling be stopped?
Many matatus leave their engine running all day. "My matatu is switched on for about 14 to 16 hours a day," says Bernard, a matatu driver who is preparing to depart for Mombasa Road in Nairobi.
At many matatu stops, drivers are actively discouraged from switching their engines off. "The police will fine and arrest you and seize your matatus of you don't keep moving when you are meant to," says Titus Muiruri, a professional driver in the city.
Several cities in Europe and elsewhere have banned motorists from idling their engines, including while they are stuck in traffic or outside schools. For instance, The British Lung Foundation launched an initiative called #DropOffSwitchOff to encourage parents and drivers of school buses to switch off their engines at the school gates. Cities like Nairobi could introduce clean air zones outside schools, hospitals and care homes where idling is banned to protect those most vulnerable.
Matatu drivers and motorists in Nairobi and beyond also need to be made aware of the costs associated with leaving engines idling. Massive savings could be made in fuel costs if drivers reduced the amount of time they left their engines idling.
A cultural change may also be needed. "If matatus and motorists in general can shift their thinking and stop their engines idling, this can have a massive effect on reducing air pollution in African cities like Nairobi," says Rob de Jong, Head of UN Environment's Air Quality and Mobility Unit.
"We must also tackle one of the root causes of urban air pollution - we need to switch from designing our cities around mobility for cars to designing our cities around mobility for people, including making it easier for those who walk and cycle - something UN Environment's Share the Road Programme is promoting."
Electric cars with the associated charging infrastructure as well as modern vehicles with stop-start systems also have a key role to play in the future to reduce emissions.
However, for the moment, the practice of engine idling in Nairobi is not changing, remaining something of a cultural norm. "I need to attract people to my business and at the end of the day, this is a hard life and I need to make money", says Samuel.
"I do know that leaving my engine is bad for people's health and a lot of people know this fact but if I don't idle my engine, then people are going to use my competitor's matatu".