When his parents died, a young Egide Kalisa was told "normal diseases" were to blame.
He's since learned the truth: respiratory disease, fuelled by indoor air pollution in his Rwandan home city of Kamembe, had more likely taken his mother and father.
Their deaths were probably caused by wood-fired cooking and heating in poorly ventilated houses.
The tragedy made Kalisa recognise air pollution - from sources ranging from car exhausts to industrial emissions - as a threat to humanity, and one that's becoming more critical each year.
Unless the world tackles climate change, deaths caused by air pollution are expected to increase by about 60,000 globally by 2030 - and 260,000 by 2100.
Now studying toward a PhD at the Auckland University of Technology, the 27-year-old has made it his mission to tackle the problem.
As the planet marks World Car Free Day tomorrow - agencies around New Zealand are encouraging vehicle commuters to take the bus or train instead - Kalisa has shared research revealing how car-less days quickly halved air pollution levels in one Rwandan city.
After losing his parents, Kalisa lived with his sister and brother, who weren't much older, and the family were forced to raise themselves.
"I became curious about my parents' death when I started high school - I would always search information on respiratory diseases, because of the pain I felt that my parents had died from it," he said.
"I found that air pollution is a major environment-related health threat to humans and a risk factor for both acute and chronic respiratory disease - and in doing so, I discovered what I really want to do in life.
"I often found myself asking the question 'why not me?' after losing my parents - I took the view that if others could achieve something, then I could too, and was fortunate in my schooling."
After gaining a scholarship from the Rwandan government, Kalisa studied air pollution at university, travelled across the world, became an assistant lecturer, and won the Commonwealth Scholarship, which brought him to AUT.
Today, he's assessing urban aerosols in New Zealand and Rwanda, in the hope his findings will help his home country's government combat air pollution.
Around Auckland, Kalisa has been measuring key air pollutants PM2.5 and PM10, and testing them for polycyclic aromatic hydrocarbons (PAHs) and their nitrated derivatives (NPAHs), which are one of the most studied families of organic compounds, given their proven carcinogenic effects on humans.
To investigate what pathogenic microorganisms also lurk in Auckland's air, he's collected data for biological aerosols, building a new methodology in the process.
While his findings showed Auckland had relatively low levels of PM2.5 and PM10 compared with other international centres, that didn't mean the city was clean.
Read more: How dirty is Auckland's air?
Heating and heavy traffic brought a high concentration of NPAHs, especially in winter.
However, with its limited amount of heavy industry, coastal location, strong winds and isolation from other continents, Auckland could boast relatively good air quality, he said.
Back in Rwanda, his research revealed a 56 per cent drop in air pollution during the country's car free days, which its government had originally brought in to encourage exercise.
In Kigali city, air quality improved just 48 hours after a car-free day.
Rwandan officials have asked for his report and recommendations; he plans to present his Auckland findings at upcoming conferences.
"Governments should invest in air pollution research and recognise that we need to take steps to be part of the solution and not part of the air pollution problem," he said.
"As Rwanda and New Zealand grow, maintaining a healthy population with clean air will be an increasing challenge.
"And with vehicle emissions having been found to be major contributors of PM2.5 and PM10 in Kigali and Auckland city, our governments should take steps to improve the public transport, and promote walking and cycling."
To join Auckland's Car Free Day event, check out the Facebook page.