Tens of thousands of local residents in the northern region of Catalonia in Spain were told to stay indoors on Thursday after an explosion at a chemical warehouse unleashed a toxic orange cloud over a wide area covering approximately six towns.

According to a spokeswoman from Catalonia’s regional fire department, the chemical blast occurred when nitric acid and ferric chloride got mixed while they were being delivered to a warehouse in the city of Igualada.

The Guardian reports:

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.”

Whether the warnings offered comfort to locals was not immediately clear. As the cloud began to dissipate, officials partially lifted the safety order but continued to advise children, seniors and pregnant women to stay inside.

This video posted by the Guardian by shows the cloud as it spread towns and highways:

Alarming photos of the cloud were posted to social media sites:

Toxic orange cloud spreads over Spain's Catalonia. #Igualada via @PressTV http://t.co/Q40HqeHsiT pic.twitter.com/37kGWRMKp1

— Amin Khorami (@aminismyname) February 12, 2015

Orange sky in #Catalonia: Chemical explosion engulfed north east #Spain towns with toxic cloud http://t.co/RQNUxRfKmZ pic.twitter.com/geXhSsiGHu

— teleSUR English (@telesurenglish) February 12, 2015

Large orange toxic cloud spreads over 5 towns after chemical blast in Spain http://t.co/q0dq9Nwt19 pic.twitter.com/g2G9Md9Si6

— Chicago Tribune (@chicagotribune) February 12, 2015