"We are struggling to breathe," said 30-year-old teacher Tayane Moraes, one of some 460,000 people who live in the city near the border with Bolivia.
On Tuesday, the concentration of cancer-causing microparticles known as PM2.5 reached 56.5 micrograms per cubic meter of air in Porto Velho -- 11 times more than the limit recommended by the World Health Organization and the worst of Brazil's big cities.
Inhaling PM2.5 has been found to increase the risk of lung cancer, heart disease, stroke, diabetes and a range of other health problems.
On August 14, the level was a "dangerous" 246.4 micrograms per cubic meter, according to the IQAir monitoring company.
It can be difficult to escape the smoke, even at home.
"It's terrible, yesterday I woke up at midnight and my eyes were tingling because of this smoke entering my house," 62-year-old retiree Carlos Fernandes told AFP.
The government of Rondonia state believes illegal fires, often started by farmers clearing land, are one cause of the disaster and has launched an online campaign calling on the population to report them.
Historic drought
According to data collected by satellites of Brazil's INPE Space Research Institute, Rondonia has just had its worst month of July for forest fires in 19 years with 1,618 confirmed outbreaks.
So far in August, there have been 2,114.
The Amazon as a whole has recorded more than 42,000 forest fires from January 1 to August 19, according to the INPE, the worst number in nearly two decades.
That number was 87 percent higher than in the same period of 2023.
The Amazon suffered a historic drought between June and November last year.
INPE's satellite images show a plume of smoke crossing Brazil from north to south, also passing through neighbors Bolivia and Paraguay.
State authorities insist much of the smoke enveloping Porto Velho, its capital, comes from fires in Bolivia, to the west, and the neighboring state of Amazonas, to the north.
"Because we are in the center of the continent, the smoke stays longer here," Cae Aires of the CENSIPAM Amazon protection center said in a video published on the Instagram account of Rondonia governor Marcos Rocha.
In the same video, infectious disease specialist Antonieta Ferreira reported "an increase in asthma attacks, as well as cases of pneumonia or sinusitis" among patients at a children's hospital.
"It's complicated with all this smoke, especially for those who have breathing problems," sighed Beatriz Graca, a 35-year-old homemaker in Porto Velho.
Forest fires have increased even as deforestation -- which helps reduce global warming by absorbing carbon dioxide -- is on the wane.
President Luiz Inacio Lula da Silva has pledged to put a stop to illegal deforestation of the Amazon by 2030.
The practice had dramatically worsened under his far-right predecessor Jair Bolsonaro.
