Two 17-year-olds from Kenya just solved vehicle air pollution with a $125 farm waste exhaust filter

Two Kenyan teenagers built a vehicle exhaust filter from coconut shells, maize cobs, and living algae. Global green tech might want to take notes.

Growing up in Naivasha, a town about 90 kilometres northwest of Nairobi, Fredrick Njoroge Kariuki developed bronchitis as a child. The air around him – thick with particulates from passing matatus, boda bodas, and ageing trucks – was the reason he struggled to breathe. “I stopped thinking of air pollution as an environmental issue,” he has says, “and started thinking of it as something being done to us.”

That distinction – between pollution as a systemic statistic and pollution as a daily assault on a child’s lungs – is the invisible thread running through HewaSafi exhaust filter, the invention that this month won Kariuki and his co-inventor, fellow 17-year-old Miron Onsarigo, the Africa regional prize at the 2026 Earth Prize competition.

The Earth Prize was founded by The Earth Foundation, a Geneva-based non-profit that emerged in 2019. The Prize, open to teenagers aged 13 to 19, offers regional winners $12,500 to implement their solutions, with a global winner receiving $100,000.

On a side-note, Hewa safi means “clean air” in Swahili.

Why Kenya’s Matatus and Boda Bodas Are at the Heart of Africa’s Air Pollution Crisis

The global conversation about decarbonising transport has, for over a decade, been dominated by a single vision: replace the combustion engine with an electric one. It is a compelling vision. However, it is also, for the vast majority of the world’s urban poor, essentially science fiction.

Consider the matatu. These shared minibuses – brightly painted, blaring music, perpetually overcrowded – are the circulatory system of East African cities. In Nairobi alone, an estimated 33,000 matatus carry roughly 3.4 million passengers daily. Add to this the boda bodas, the motorcycle taxis that weave through gridlocked streets carrying everything from office workers to sacks of grain, and you have the backbone of mobility for tens of millions of people across Kenya, Uganda, Tanzania, and beyond.

These vehicles are old. They run on diesel and petrol. Their owners are small operators – often a single family’s livelihood depends on one vehicle – with no capital for a KSh 4 million electric replacement. They are not going away. They are, in fact, the primary reason Nairobi ranked among the ten most polluted major cities in the world as recently as August 2025, with road transport responsible for around 40% of the city’s PM2.5 concentrations. The World Health Organisation estimates that air pollution kills approximately 19,000 Kenyans every year.

The global green transition has not come for the matatu. So two teenagers decided they would and they created their own cheap exhaust filter, the HewaSafi.

How the HewaSafi Exhaust Filter Works: The Farm Waste Exhaust Filter That Cuts Emissions by 93%

The HewaSafi exhaust filter is a functional exhaust filtration system built from materials that could, in principle, be sourced from a Kenyan market: coconut shells, maize cobs, steel mesh, copper, components salvaged from discarded batteries – and, in one of its most quietly remarkable features, a chamber of living spirulina algae.

The prototype cost approximately KES 16,288 to build. That is around $125. Check this website to see a 3D model of the system.

For context, conventional catalytic converters and emissions-control systems used in wealthier markets rely on platinum-group metals – palladium, rhodium, platinum – that must be mined, refined, and imported. Their costs run into thousands of dollars. They are designed for, and priced for, the markets of Europe, North America, and East Asia. For matatu owners, these exhaust filters are not really an option due to the price.

HewaSafi achieved a 93.3% reduction in PM2.5 particulate emissions in testing – far exceeding the inventors’ own targets. It also reduces toxic carbon monoxide and CO2 output. It was designed specifically to fit vehicles like matatus and boda bodas, which means it was designed with the specific pipe gauges, engine ages, and maintenance realities of African urban transport in mind.

The spirulina component deserves particular attention. Unlike the carbon-capture exhaust filters that bookend it, the algae is alive – a biological stage in what is, at its core, a hybrid engineered-ecological system. Spirulina is a cyanobacterium with exceptional CO2 absorption capacity, already cultivated widely in East Africa as a food supplement. Kariuki and Onsarigo are not the first to investigate algae’s potential in emissions control, but they may be among the first to integrate it into an affordable, field-ready filtration system designed for informal transport in the Global South.

HewaSafi does not require a charging infrastructure that doesn’t exist. It also does not require a government subsidy programme. And it does not require a matatu owner to take out a loan larger than their annual income. It requires, roughly, what you might spend on a month of groceries for a small family.

How the HewaSafi Exhaust Filter Compares to Existing Commercial Emissions Solutions

In the table below we compare the HewaSafi exhaust filter against the four main commercial emissions-control technologies currently available for combustion-engine vehicles.

The table compares HewaSafi across 4 solutions – standard catalytic converter, DPF, DOC, and three-way catalytic converter – on 12 dimensions including cost, materials, PM2.5 reduction, retrofit complexity, and local sourceability. You will notice that the DPF is the most effective commercial option, but it physically can’t work on the old engines in most East African matatus. HewaSafi’s passive design is what makes it viable where everything else isn’t.

	HewaSafi	Standard Catalytic Converter	Diesel Particulate Filter (DPF)	Diesel Oxidation Catalyst (DOC)	Three-Way Catalytic Converter
Unit cost	~$125 (KES 16,288)	$200–$2,000	$2,500–$12,000	$300–$800	$500–$2,000
Key materials	Maize cobs, coconut shells, recycled batteries, spirulina algae, copper, steel mesh	Platinum, palladium, rhodium (imported)	Cordierite or silicon carbide ceramic, platinum-group metals	Platinum, palladium on ceramic substrate	Platinum, palladium, rhodium
PM2.5 reduction	93.3%	70–90%	85–99%	20–50%	70–90%
CO / CO2 reduction	Yes	Yes (CO, HC, NOx)	Limited (targets soot)	Yes (CO, HC)	Yes (CO, HC, NOx)
Target vehicles	Matatus, boda bodas, older petrol/diesel fleet	Modern petrol vehicles	Modern diesel vehicles	Diesel vehicles	Modern petrol vehicles
Requires imported components	No	Yes	Yes	Yes	Yes
Locally sourceable in East Africa	Yes — all materials available regionally	No	No	No	No
Designed for African informal transport	Yes	No	No	No	No
Retrofit complexity	Low	Medium	High (requires specialist fitting)	Medium	Medium
Maintenance requirement	Exhaust filter replacement + spirulina cultivation	Periodic inspection	Regeneration cycles, cleaning every 60,000+ miles	Periodic inspection	Periodic inspection
Cost per tonne of PM2.5 reduced	Est. very low (under $500)	Moderate	$18,700–$87,600 (EPA data)	Moderate	Moderate
Works on older/ageing engines	Yes	Partial	No (requires compatible engine management systems)	Partial	No

Date sources for this table:

• Catalytic Converter Price Guide 2025 – RRCats
• DPF Replacement Cost – AutoNation Mobile Service
• DPF Replacement Cost for Heavy Duty – Heavy Duty Journal
• Retrofitting Gensets with DPFs – Rypos
• Diesel Retrofits cost-effectiveness – Federal Highway Administration

Earth Prize 2026 Global Vote: What Happens Next for HewaSafi?

Six of the Earth Prize’s eight regional winners have now been announced – Africa, Europe, the Middle East, Asia, Oceania, and Southeast Asia – with two regions still to be revealed. A public vote opens on May 18, with the global winner to be announced on May 29.

Whether the HewaSafi exhaust filter wins the global prize or not, the more important question is what happens to it in the real world. $125 per exhaust filter unit – even if that cost holds at scale, which it may not – still requires someone to manufacture, distribute, fit, and maintain the devices. Matatu owners need to be persuaded. City regulators need to be engaged. The spirulina needs to be cultivated and the exhaust filters need to be replaced.

None of this is simple. But the proof of concept exists, and it exists at a price point that makes it theoretically deployable at the scale at which the problem actually exists, and that is across the tens of thousands of ageing combustion engines that constitute the daily reality of African urban transport.

For now, Fredrick Kariuki and Miron Onsarigo are still 17, still in school, and have just won an international environmental prize.

Hewa safi. Clean air. For the matatu. For the boda boda. For the kid in Naivasha who couldn’t breathe.

The Earth Prize public vote opens May 18 at theearthprize.org/vote.

Sources:

• 17-year-old Kenyans named Africa $100K Earth Prize 2026 winners – BiznaKenya
• Teen innovators in Kenya turn farm waste into award-winning vehicle exhaust filter – Mongabay
• HewaSafi by two Kenyan Teenagers Wins $100K at Earth Prize – TechMoran
• Two Kenyan High School Students Win KSh 1.6m Earth Prize – Tuko.co.ke
• Nairobi matatus’ odd engine idling culture pollutes, harms health – UNEP
• Nairobi and air pollution – Clean Air Fund
• Nairobi among top 10 most polluted major cities, August 2025 – IQAir
• Earth Prize microplastic solution: Teen wins Europe award for Eco Purge – The European

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