That's according to Stockholm Air & Noise Analysis (SLB-analys), the authority responsible for monitoring air quality in the city, which observed particularly dirty air at their Hornsgatan and Häggvik stations in the capital last Sunday.
“The inward movement of dirty air from central Europe caused very high levels of particles in Stockholm's air on Sunday. The situation worsened further because of the weather,” Malin Täppefur from SLB-analys told The Local.
“The high concentration occurred for around eight hours and was as high as in Beijing. The difference between Beijing and Stockholm however is significant. Beijing has very high levels for long periods. In Stockholm it's extremely unusual, and it doesn't even occur annually,” she added.
The dangerous microscopic particles in the air can be caused by wear and tear on road surfaces, tyres and vehicle brakes, but above all weather conditions were to blame. A relatively low level of precipitation in the city meant that cars with winter tyres swirled the dust up from the road and into the air, creating a high level of particles.
“The weather conditions hampered the dilution and purging of the pollution. There were very high levels,” Täppefur noted.
According to the expert, there is little the city could have done to prevent the unusually smoggy day, and instead, other neighbouring countries with high levels of pollution were the source.
“Stockholm can't affect the air which is transported in. Measures must be taken in countries like Poland, which is one of Europe's most polluted countries,” she concluded.
Air problems of this kind normally only appear in Sweden later in the year around the spring, when the snow has melted.