São Paulo, the world’s 5th largest metropolis, has been found to be home to a surprise source of air pollution in spite of the city’s efforts to curb vehicle emissions, a new study states.

Housing one tenth of Brazil’s population with 21 million residents, São Paulo has been struggling with severe air pollution issues, largely stemming from the 7+ million motor vehicles trawling its roads. The city has taken positive steps to address this problem, making it mandatory for the city’s fleet to run entirely on green biofuel, composed of sugarcane ethanol, gasohol and soya diesel, which reduces harmful vehicle emissions.

In spite of these progressive steps, a study published in Atmospheric Environment has suggested another contributor to the city’s emissions is more significant than previously realised: the wood-fired stoves used in pizzerias.

Whilst feijoada (a pork and bean stew) is often considered Brazil’s national dish, São Paulo has developed a remarkable appetite for pizza. Visiting the neighbourhood pizzeria has become a family staple on Sundays, with people queuing up for hours to get a slice. The city is now home to around 8000 pizzerias, some seating up to 600 people at once; a press release states that 800 pizzas per day are cooked using the traditional wood-burning stoves, plus 1000 more for delivery each day, with Sundays the busiest day of the week.

Wood-burning stoves are favoured for their very high temperatures, quick cooking times, and the ability to give pizza its authentic blackened, delicious crispiness.

The study, co-authored between air pollution experts from seven universities and led by Dr Prashant Kumar of the University of Surrey, found that the levels of pollution resulting from this wood-burning provided cause for concern on numerous counts.

Kumar commented, "There are more than 7.5 hectares of Eucalyptus forest being burned every month by pizzerias and steakhouses. A total of over 307,000 tonnes of wood is burned each year in pizzerias. This is significant enough of a threat to be of real concern to the environment negating the positive effect on the environment that compulsory green biofuel policy has on vehicles."

A co-author, Professor Yang Zhang of North Carolina State University notes the public health threat posed by this kind of pollution: the particles emitted contribute towards ozone and secondary aerosols in the city. Whilst much focus has been placed on the effect of vehicle emissions on human health, Zhang emphasises how other forms of pollution such as from wood-burning have so far been less quantified.

Fortunately, there are methods to minimise the polluting impact of this style of cooking: restaurants can install filters or catalytic exhaust systems, a precaution that was already imposed upon pizzerias in this small town in Italy to curb pollution.