Pregnant women, children, elderly people and those with respiratory problems were subject of special warning . The recommendation to stay indoors was partially withdrawn an hour later. Igualada is located some 40 miles northwest of Barcelona. The orange cloud was visible on a large area. The officials concluded it was an accident. The human error was involved. Nitro compounds are shock- and thermally-sensitive explosives. Exposure to nitric acid vapor can cause problems if it is inhaled, swallowed or comes into contact with the eyes or skin. It is usually just an irritant, commonly affecting the eyes, throat and skin but higher doses can cause severe burns.