Andean cities choked by traffic pollution

Low oxygen levels and steep streets are not good for healthy mobility!

“At an altitude of 4,000 metres, there is 40% less oxygen in the air, leading to incomplete combustion in car engines, which produces copious amounts of soot”,

explains Gaëlle Uzu, an Atmospheric Geochemist at IRD’s Institute of Environmental Geosciences (IGE). With Bolivian partners from the Universidad Mayor de San Andrés (UMSA), in the Atmospheric Physics Laboratory, Gaëlle Uzu’s team spent two years analysing the sources of fine particle emissions [1] in La Paz, their dangerousness and their effects on the health of local residents.

Hill starts and breaking

Like many cities in the Andes, Bolivia’s economic capital sprawls over hilly terrain, with a 600-metre difference in altitude between the suburbs and the highest districts. This is good for air quality, as pollutants are dispersed by the breezes that rise up the slopes during the day when it is warm, thanks to the updrafts. However, this advantage is largely swept away by traffic on the inclined streets.

“In addition to exhaust fumes, congestion in steep streets causes frequent starting and stopping, which significantly increases vehicle emissions, as the abrasion of tyres and brakes releases numerous particles into the atmosphere”,

explains the specialist. Other high-altitude cities, such as Bogotá and Quito, could also suffer significant traffic-related air pollution, from both soot and wear.

Non-tarmacked streets and dust

Analyses of fine particles in the atmosphere of La Paz also revealed large quantities of resuspended mineral dust. This source of pollution could be partly linked to traffic on the city’s many non-tarmacked roads.

“Above all, our work has shown that the particles emitted by vehicles in this context, which differ in their compounds from those emitted in cities at lower altitudes with fewer hills, are potentially harmful to the body",

says Valéria Mardonez, an UMSA PhD student at the IGE. In vitro experiments showed that they have a high oxidative potential, meaning that they can damage cells, particularly those in the lungs, accelerating ageing and increasing the risk of cancer, cardiovascular and neurodegenerative diseases.

A new indicator

In fact, these noxious particles have very real effects on the health of the population of La Paz: the study showed that peaks in emissions linked to road traffic and dust were followed a week later by an increase in hospital admissions for acute respiratory infections and pneumonia.

“Over the two years of the study, it was shown that there was a greater correlation between respiratory ailments and the dangerousness of the compounds emitted (measured by their oxidising potential) than between these diseases and the overall quantity of fine particles in the air”,

says Gaëlle Uzu. The scientists therefore recommend that the oxidising potential of particulate matter should henceforth be used as an indicator of air quality, in addition to the current indicator of particulate mass concentration. Their recommendations may well be heard by the WHO.