Polymers, which are not fully saturated, are vulnerable to attack by ozone. This gas exists naturally in the atmosphere but is also formed by nitrogen oxides released in vehicle exhaust pollution. Many common elastomers (rubbers) are affected, with natural rubber, polybutadiene, styrene-butadiene rubber and NBR being most sensitive to degradation. The ozonolysis reaction results in immediate chain scission. Ozone cracks in products under tension are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, and fuel leakage and fire may follow. The problem of ozone cracking can be prevented by adding antiozonants.