Milan was under a blanket of smog on Tuesday as a row broke out over data from a monitoring group that ranked the northern Italian city long known for its poor air quality among the most polluted in the world, alongside Dhaka, Lahore and Chengdu. The controversy began on Sunday, when the Swiss real time air quality website IQAir labelled Milan “unhealthy” as the level of fine particulate matter, or PM 2.5, in the city’s air was 24 times above the limit recommended by the World Health Organization (WHO), ranking the city third after Dhaka in Bangladesh and Lahore in Pakistan. The densely populated industrial hub crept up to second place on Monday, after Chengdu in China, before slipping to 10th on Tuesday. According to IQAir’s website, its data is gathered from “governmental stations and low-cost sensors owned by citizen scientists around the world”. The Milan mayor, Giuseppe Sala, who has introduced some bold anti-pollution measures since being elected mayor in 2016, has dismissed the IQAir data as “the usual impromptu analyses made by a private body” and accused journalists of reporting from “social media”. “I’m really annoyed at having to ask questions that don’t exist,” he said on Monday. The Lombardy unit of Arpa, a regional environmental protection agency, described the IQAir classification as “unreliable” but confirmed that the city’s air had eclipsed a PM 2.5 limit in recent days, triggering a series of anti-pollution measures, including limits on daytime traffic, to be enacted in Milan and eight other cities in the badly affected areas of the region. Lombardy is in the Po Valley, a huge geographic area in northern Italy which is among the worst in Europe for air pollution. A Guardian investigation last year found more than a third of the people living in the valley and surrounding areas breathed air four times the WHO’s guideline limit for the most dangerous airborne particulates. The issue in recent days has been exacerbated by unseasonably high temperatures, and should subside with the arrival of rain from Thursday, according to Guido Lanzani, the director for air quality at Arpa Lombardy. He said that while the region was experiencing a “critical period for air pollution”, the agency “cannot confirm what was published by IQAir”. He told LaPresse news agency the data on the site “varies from hour to hour” and comes from “very different types of sources”. Legambiente, Italy’s largest environmental organisation, accused Lombardy’s leaders of having no clear strategy to improve air quality, while more than 50,000 residents in Milan have reportedly backed a push to take legal action against the city’s authorities over the smog levels. “I don’t know what made Sala express himself in that way, I won’t go into the merits of the data,” the lawyer Bruno Borin told the Adnkronos news agency. “The time has come for a class action lawsuit. We can’t wait until we get sick.” Poor air quality was linked to 50,303 premature deaths in Italy in 2020, according to the EEA. Most were in Milan, but Cremona in Lombardy was the Italian province with the highest proportion of deaths – between 150 and 200 per 100,000 residents – attributed to PM 2.5.