Two 17-year-old Kenyan students have developed a low-cost vehicle exhaust filter dubbed HewaSafi that reduced dangerous fine-particle emissions by 93.3% during early road tests on Nairobi matatus.
Fredrick Njoroge Kariuki and Miron Clifford Onsarigo, students at the M-PESA Foundation Academy in Kiambu County, built the device from maize cobs, coconut shells, metal components, recycled battery materials and living Spirulina algae. They named it HewaSafi, Kiswahili for clean air.
The HewaSafi prototype costs about KSh16,288, or roughly $125, substantially less than the approximately KSh50,000 price cited for some existing commercial systems. That cost difference is central to the project. Pollution-control equipment that is too expensive for matatu owners and other small transport operators is unlikely to achieve broad adoption in Kenya or other African markets dominated by ageing, second-hand vehicles.
HewaSafi channels exhaust through five filtration compartments, with each stage intended to capture or reduce a different pollutant. Carbon-rich maize cobs and coconut shells serve as locally available filtration materials, while steel mesh, copper and recycled battery parts form part of the mechanical system. A Spirulina algae chamber is used for bioremediation, with the living organism intended to absorb part of the carbon dioxide passing through the device.
The students tested prototypes on five matatus operating along Nairobi’s Thika Road corridor. Sensors collected readings at six-hour intervals under normal road conditions. The results reported by the team showed a 93.3% decline in PM2.5 emissions, a 42% reduction in carbon monoxide and 21.4% direct carbon dioxide capture. PM2.5 refers to particles small enough to penetrate deep into the lungs and enter the bloodstream.
Long-term exposure is associated with respiratory and cardiovascular diseases, including stroke, lung cancer and chronic obstructive pulmonary disease.
For Nairobi, the choice of public service vehicles, commonly known as matatus, was deliberate. The privately operated minibuses carry a large share of the city’s commuters, but congestion, prolonged engine idling and older diesel engines make them a visible source of roadside pollution. UNEP has previously warned that thousands of idling matatus and other vehicles release harmful particles into areas where passengers, drivers and street traders face direct exposure.
Kariuki grew up in an industrial area of Nakuru and developed a chronic lung condition at the age of ten. He has said the illness still requires weekly medication. Onsarigo, who grew up in western Kenya, had also witnessed serious illnesses and deaths linked to polluted air. Their shared experience turned the project from a school experiment into an attempt to build a device suited to Kenyan roads and household incomes.
The HewaSafi team later partnered with a matatu association to move testing beyond the laboratory. That step gave the students early performance data under real driving conditions, where heat, vibration, soot buildup and changing engine loads can expose weaknesses that may not appear in controlled experiments.
HewaSafi won the Africa regional category of The Earth Prize 2026, an international environmental competition for students aged between 13 and 19. The award brought the team $12,500 in funding, which the students plan to use for further testing, design improvements and development of a version that can be manufactured at scale.
The judges cited the project’s combination of agricultural waste, accessible materials and a functioning technical prototype. Rather than presenting a theoretical proposal, the students had already built the filter, installed it on vehicles and collected field data.
The use of maize cobs and coconut shells also gives the project a local manufacturing advantage. Both materials are widely available as agricultural waste in Kenya and elsewhere in Africa. Converting them into filtration media could reduce dependence on imported components while creating an additional market for waste that would otherwise be burned, dumped or left to decompose.
The reported reductions are substantial, but HewaSafi remains an early-stage prototype. The road trial involved only five vehicles, and the published figures come from the project team rather than a large independent engineering study. Before commercial use, the device would require longer trials across different engines, fuels, routes and vehicle ages.
Engineers would also need to establish how frequently the filtration materials must be replaced, whether the system affects exhaust pressure or fuel efficiency, how it performs after months of soot accumulation and how safely recycled battery components can be incorporated. The carbon dioxide claim will require particular scrutiny. Capturing carbon from hot, fast-moving exhaust gases is technically difficult, and a living algae component must remain viable under changing temperatures and operating conditions.
Independent laboratories would need to verify how much carbon is retained permanently rather than temporarily absorbed and later released. Certification will be equally important. A retrofit device fitted to public-service vehicles must meet mechanical, emissions and road-safety standards before regulators and fleet operators can approve widespread installation.
Kenya is expanding electric mobility, but replacing the existing vehicle fleet will take years. Most matatus, buses, trucks and private cars currently on the road will continue using petrol or diesel during that transition. Heavy-duty vehicles are especially important to urban air-quality policy.
UNEP estimates that they account for more than 60% of on-road PM2.5 emissions and over 40% of on-road nitrogen oxide emissions globally, despite representing a smaller share of total vehicles. Affordable retrofit technology could therefore play a supporting role alongside electric vehicles, cleaner fuels, emissions inspections and the removal of heavily polluting vehicles from service.
HewaSafi will not solve Nairobi’s air-pollution problem on its own. Its commercial value will depend on independent validation, regulatory approval, durability and whether transport operators consider the running costs manageable. The students have nevertheless developed a working response to a problem common across African cities. At about one-quarter of the price quoted for some existing alternatives, their prototype offers a testable route towards cleaner exhaust emissions without waiting for every vehicle on the road to be replaced.
