Pennsylvania ? A tractor-trailer carrying a dangerous acid overturned on a highway Saturday, prompting authorities to order thousands of residents to leave the area for almost nine hours.

The tanker, carrying 33,000 pounds of corrosive hydrofluoric acid, a component for household detergents, flipped on a sloping curve in the road at about 3 a.m. on the edge of Wind Gap, about 60 miles north of Philadelphia, and began leaking slowly.

Hydrofluoric acid in low doses can irritate the eyes, nose and respiratory tract, and in higher doses it can cause severe burns, chronic lung disease or even death, the Centers for Disease Control and Prevention says.

At 7 a.m., officials ordered an evacuation of 944 households. They said about 5,000 people were in the affected area.