A flower show in the gritty Chinese industrial city of Tangshan has emerged as a major cause of the spike in iron ore prices this week, as steel mills rushed to buy before being forced to close to keep skies blue for 10m expected visitors.

Steel mills in the city, located north-east of Beijing in Hebei province, have been told to shut down from late April to October to reduce pollution during an international horticultural exhibition. The city at the weekend urged its steel mills to increase production in anticipation of the shutdown, sparking a scramble for the key steelmaking ingredient.

Temporary air quality control measures implemented for the event will halve production at local mills until the end of September, said Jiao Yanlong, Tangshan’s Communist party secretary, citing the Tangshan Iron and Steel Association.