Some 60,000 people living in northern Spain were urged to shut their windows and stay indoors on Thursday after a chemical explosion sent a large orange cloud floating above six towns in central Catalonia.

For most residents, the warning was lifted a few hours after the cloud appeared. Pregnant women, children, the elderly and people with respiratory problems were still urged to stay indoors, but those restrictions were lifted early Friday.

The thick cloud could be seen above the the site in Igualada before it spread over residential areas, lingering for several hours.

A spokesman for Catalonia's interior ministry told reporters that the chemicals were irritants and were not dangerous. "Most likely it will cause itchy eyes and a runny nose, but no choking sensation," he said.

As well as advising local people to stay inside, Catalan authorities closed roads and a rail line leading into Igualada.

The regional government of Catalonia said that the blast at a facility owned by Spanish company Simar was apparently caused by two volatile chemicals accidentally coming into contact when they were delivered to a warehouse.

According to the civil protection department, the chemicals included nitric acid and ferric chloride, which caused the cloud's bright orange color.

Six people were injured in the incident and one remained hospitalized Friday with leg burns, the Associated Press reported.