More than 60,000 residents in north-eastern Spain have been told to stay indoors after an explosion at a chemical warehouse sent a dense orange cloud into the sky that hovered over their municipalities for hours.

As the cloud settled over of six municipalities in central Catalonia, including Igualada, Jorba and Òdena, Spain’s emergency services told residents to close their windows and seal off any means of ventilation. “This is not a game,” they tweeted. “Don’t put yourself in danger to take pictures of the cloud outside.”

Initial reports had described the cloud as toxic, but Ramon Espadaler of Catalonia’s interior ministry later told reporters that the chemicals were simply irritants. “Most likely it will cause itchy eyes and a runny nose, but no choking sensation.”

Firefighters said the blast was caused when the contents of a delivery – reportedly including chemicals such as nitric oxide and ferric chloride – became mixed as they arrived at a warehouse in Igualada on Thursday morning.

Two people were slightly injured, and three of the firefighters who were first to the scene needed treatment for minor ailments, authorities said.

About two hours after the explosion, emergency services lifted the advisory but said children, pregnant women, elderly people and anyone with respiratory illnesses should remain in their homes.

The cloud had dissipated into a fine mist and was expected to dispel soon, Espadaler said, noting that the situation had been downgraded from an emergency to an alert. “There’s very little wind, which means the cloud is disappearing very slowly,” he said. “We’re returning to normal little by little, but we still don’t have everything under control.”