Fredrick Njoroge Kariuki was ten when the air in his home town gave him a lung condition he still lives with, and Miron Onsarigo grew up in western Kenya watching people he knew fall ill from the same thing. At the M PESA Foundation Academy in Kiambu County, the two of them decided to build the part of the answer they could reach: the exhaust pipe. Their filter, called HewaSafi, which means clean air in Swahili, divides the exhaust into five chambers, each one built to catch a different pollutant. The chambers are packed with things a Kenyan farm throws away, maize cobs and coconut shells turned into charcoal, plus steel mesh, copper and, in the final stage, living spirulina algae that drink in carbon dioxide as the gas passes through. In testing the prototype removed more than ninety three per cent of the fine soot that does the most harm to lungs, cut carbon monoxide by forty two per cent and absorbed about a fifth of the carbon dioxide.
The price is the other invention. A conventional filter in Kenya costs around fifty thousand shillings, about three hundred and ninety US dollars, which is why the minibus taxis and motorbike taxis that move most of East Africa tend not to have one. HewaSafi comes in at just over sixteen thousand shillings, around a hundred and twenty six dollars, and it is built for exactly those vehicles. The pair have already fitted it to five matatus on Nairobi's Thika Road and are working with the matatu owners' association on a trial of two hundred vehicles, a network of around eight thousand drivers. In May the Earth Prize, the world's largest environmental competition for teenagers, named them its Africa winners and handed over twelve and a half thousand dollars to build the next batch. One of the judges described it as a tangible technical pathway using locally accessible materials, which is the engineer's way of saying the best ideas are made from what is lying around.
Questions you might have
Can I buy a HewaSafi filter?
Not yet. It is a prototype being trialled on five minibuses in Nairobi, with a two hundred vehicle trial planned. The prize money is for building the next batch.
Why maize cobs and coconut shells?
Turned into charcoal they become a honeycomb of tiny pores that trap soot, the same idea as the activated charcoal in a water filter, and in Kenya they are free and everywhere.
What is the algae for?
The last chamber holds living spirulina, which takes in carbon dioxide from the exhaust as it passes. It is the part of the filter that is actually alive.
Our short take: the full story is by Malavika Vyawahare and Mary Mwendwa for Mongabay, 12 May 2026. Prize details from The Earth Prize. Photograph: Ephymbaya, CC BY-SA 4.0.
