Complex sentence: When high levels of dangerous lead contamination were recognized in the town of Herculaneum, Missouri in 2001, residents contacted the MCE, who began to monitor the situation in addition to smelter emissions.
Split into simpler sentences: When high levels of dangerous lead contamination were recognized in the town of Herculaneum in 2001, residents contacted the Missouri Coalition for the Environment. MCE began to monitor the situation in addition to smelter emissions.