The volunteers risking their lives to defuse Indonesia's wildfire 'carbon bombs'

The sky grows heavy in Borneo, though not with rain. The air, amber-hued and treacle-thick, is smudged under a blanket of smog that has spilled across international borders – to Singapore, Manila, Kuala Lumpur.

"It hits your nose straight away, your eyes burn – that's more or less what it's like," he says. "People say these fires are extraordinary because of the flames and the smoke. And they're right."

The 35-year-old volunteer firefighter has spent more than a month enveloped in this haze, breathing toxic millennia-old microparticles of carbon as he toils against a seemingly inextinguishable fire front.

Using hand tools, carts and portable fire pumps, he and thousands of other volunteer firefighters – many of them indigenous Dayak or Banjar people – are up against an inferno that has carbonised some 200,000 hectares of forest and farmland across Indonesian Borneo since early July.

Underground and out of sight, giant subterranean peat fires continue to smoulder, choking the air with poisonous fumes.

"Once a fire becomes uncontrolled, it harms many people. Not just humans, but also animals and plants," Sujatmoko says. "If underground fire spreads and can no longer be controlled, the losses are enormous."

Amid a so-called "Super El Niño" that has supercharged local drought conditions, thousands of active fire hotspots are spreading and multiplying across the country, fuelling a transnational haze crisis that is the region's worst in more than a decade.

But what makes Borneo's fires particularly worrying is the fact that they are burning through one of the world's largest tropical peatland ecosystems.