Noluthando Geja isn’t looking forward to summer.

The unemployed 44-year-old dreads the fast-approaching hottest months of the year, when the blistering sun beats down on the steel-sheet roof of the two-room house she shares with three children in Itireleng, a shantytown wedged between Johannesburg and South Africa’s capital, Pretoria.

“Do you see how little there is?” Geja says of the space as she stands in her tiny kitchen. “You find sometimes you are stressed and you’re shouting at them unnecessarily because of the heat,” she says of the children.

Itireleng lies at the heart of vast urban sprawl of about 12 million people that includes Pretoria and Johannesburg, the biggest city and commercial center.

Geja’s home nestles in a hodgepodge of tightly packed, rudimentary brick structures and cobbled-together tin shacks dotted with the odd scrubby tree. But just 25 miles to the east, in Pretoria’s Waterkloof, spacious and verdant gardens face onto streets lined with purple-flowering jacaranda trees. A similar distance to the south, in Johannesburg’s affluent northern suburbs, oak and London plane trees shade ample gardens in what affluent locals like to describe as one of the world’s largest urban forests.

Such extremes illustrate a dichotomy that exists across this country. The wealth gaps are obvious, but less so are the differing levels of heat stress that the rich and poor experience, according to a study earlier this year for the World Bank and South Africa’s National Treasury. The Thomas Piketty-backed World Inequality Lab found inequality in South Africa to be the world’s most unequal country for which data is available.

The disparity can be traced back to apartheid, when urban planners kept Black South Africans in poorly developed townships on the margins of cities where little attention was paid to providing anything more than the most basic services. Added to that are the shantytowns that abut them, with many of the country’s rural poor and undocumented migrants from other countries erecting densely packed, makeshift shelters that trap heat.

By contrast, the largely treeless plains that are the natural habitat in the region have, in the case of the more affluent areas of Johannesburg, been heavily planted with shady exotic trees from temperate regions such as Europe. Today the city is home to more than 10 million trees and has more than 2,000 public parks, according to an assessment by The Nature Conservancy, a US-based conservation nonprofit.

Older parts of Johannesburg boast tree cover of more than 80%, with many planted to provide shade, according to Jason Sampson, head of the University of Pretoria’s Botanical Gardens department. Waterkloof has 54.1% tree cover and almost 10 times the median income as that of Mamelodi, a township on the outskirts of the city, where the tree cover is just 2.6%

“They’re generally quite big. They’ve got a big spread,” Sampson said of the shade trees. “The lack of trees in informal settlements is structural inequality in a different way. Informal settlements don’t have the luxury of pre-planned green infrastructure.”

The few trees that are found in shantytowns rarely survive.

“The chances are very good that people will chop them down for firewood rather than keeping them,” said Carin Combrinck, an architecture professor at the University of Pretoria.

Geja participated in the study — a so-called heat-mapping campaign — where 58 local community members in the municipal areas of Cape Town, Buffalo City and Tshwane (where Itireleng lies) traveled from shantytowns into affluent suburbs using temperature sensors attached to vehicles. The area stretches about 80 miles north to south and 60 miles east to west across the Gauteng province.

What they found was that townships were 6 degrees Celsius hotter than wealthier suburbs, while indoor temperatures in makeshift shacks were 8 degrees Celsius higher. Temperatures in those structures can rise to as high as as 48.5 degrees Celsius, according to a separate study by the University of Pretoria’s Architecture and Chemical Engineering departments in the nearby shantytown of Melusi.

“When the sun’s rays beat down across the city as a whole, those neighborhoods with less tree cover don’t see that benefit of evaporative cooling,” said Nicholas Jones, a data scientist at the World Bank’s global facility for disaster reduction and recovery. “They see more of the energy from the sun being stored in the surfaces and re-emitted at night, heating up the bodies of people who live there, making it harder to get your day’s work done and really adding to hardship.”

During much of December, indoor heat conditions present “danger” between 73% to 93% of the time, and in South Africa’s hottest month of January indoor conditions quickly turn to being extremely dangerous, the researchers wrote. In Melusi, where about 27,000 people live, the population density is about 160 people per hectare (2.47 acres), about seven times that of nearby suburbs. While that’s lower than Manhattan in New York, there are no air conditioners.

The consequences are lower productivity and higher heat-related mortality in the poorer parts of the cities. At more than 35 degrees Celsius, mortality in the region rises as much as 20% from a baseline of 20 degrees to 25 degrees, the researchers wrote. Heat stroke, dehydration and heat exhaustion can lead to deaths, while hot temperatures exacerbate respiratory problems.

While the area isn’t the only one to suffer from heat stress, the extremes among its inhabitants are among the widest. The heat differential between city neighborhoods matches that of Mumbai and Madrid and is well ahead of New York and Melbourne, a study published by the London-based Arup collective of designers, engineers and technical experts found.

South Africa’s National Treasury said it is giving cities money to build more brick structures, which are cooler, and promoting other measures that would provide shade and cooler temperatures in the most-affected areas.

Still, it’s hard to ensure that building standards are met.

“There are measures in place that we as a city adopt in our buildings,’’ said Daniel Sullivan, director for risk and resilience, future planning and resilience for the City of Cape Town, which formed part of the study. “In the informal space, where people are building without taking things through an approval process, those are the corners that sometimes get cut, and the implications of that on health are quite significant.”

The study recommends “strategic greening of underserved areas,” painting roofs white and creating “cool spaces” for communities. For now, Geja said, little is being done.

“When it’s hot it’s hard to work,” she said. “In tin shacks, it becomes really difficult.”