Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Gasoline and oils are complex chemical mixtures designed to burn in a way that will efficiently produce energy while emitting a minimal amount of air pollution. The refining of gasoline has improved engine performance but is much more complicated than simply using the crude products extracted from oil wells, as was common in the late 1800s. Most gasoline contained lead at one time, because this additive helped the engine run more smoothly. However, this caused lead contamination in the environment, so new “unleaded” formulations were created that could be burned smoothly without the addition of poisonous heavy metals. Oils for lubrication have special additives that reduce engine wear. Some special fuel blends have also been created to generate more power in race car engines.


Question: Why did people stop adding lead to gasoline?
environmental pollution
Due to environmental contamination concerns.