Hazardous waste -- often from Italy's wealthy north -- has long been set alight or buried in the area around Acerra, a city near Naples, in the southern region of Campania.
For decades, the soil, groundwater and air have been contaminated by heavy metals, dioxins and asbestos.
Cancer rates among the area's three million or so residents are higher than the national average.
Leo condemned "a deadly mix of obscure interests and indifference toward the common good, which has poisoned the natural and social environment".
The US-born pope drew excited crowds when he arrived in his popemobile at a piazza in Acerra, a city of around 60,000 people.
"The pope is maybe the only person who can awaken the conscience a little bit of all the people who have harmed this territory," local worshipper Giuseppina De Francesco, 60, told AFP.
In 2025, Europe's top rights court ruled that Italy had failed to protect residents and gave the government two years to fix the situation.
The pontiff's visit coincides with the 11th anniversary of a landmark climate manifesto by Leo's predecessor, Pope Francis.
The "Laudato Si" encyclical, which denounced mankind's ruthless exploitation of the environment, was hailed by experts for its scientific grounding.
'Poisoning'
"In life, we understand that the more fragile beauty is, the more it requires care and responsibility," said Leo in an address at the city's cathedral to the clergy and families of the victims of environmental pollution.
"This land has paid a heavy price. It has seen many of its children buried. It has borne witness to the suffering of children and innocents," added the leader of the world's Roman Catholics.
He thanked the "pioneer" activists he said had raised awareness with "their courageous commitment" to fight the "poisoning" of the land.
Italy's "Land of Fires", also known as the "Triangle of Death", has served as a dump and illegal incineration site since the late 1980s.
Instead of paying exorbitant sums to have toxic substances disposed of legally, companies paid the region's Camorra mafia a fraction of the cost to dump waste ranging from broken sheets of asbestos to car tyres and containers of industrial-strength glue.
Since 2013, a host of parliamentary inquiries has found the authorities negligent and in some cases complicit.
They have also highlighted the health fallout, including an increase in cases of cancer and foetal and neonatal malformations.
In 2018, the Senate said mobster criminality and political inaction had caused an ecological disaster in the region.
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