Where it's a problem: Las Vegas, Nevada; Jacksonville, Florida; and Mesa, Arizona
What it does: Bromoform is another byproduct that occurs when chlorine is added to the water supply, though small amounts are formed naturally by ocean plants. At high enough levels, it can cause sleepiness or dizziness, and at extremely high levels could knock you out or kill you. You don't want to drink too much bromoform, basically.
Where it's a problem: Los Angeles, California
What it does: Tetrachloroethylene, also known as PERC, is a manufactured substance used for dry cleaning and metal degreasing, but hey, sometimes it's in water, too! Long-term exposure can affect your central nervous system, causing changes in mood, memory, attention, vision, and reaction time.
Where it's a problem: Houston, Texas; Anaheim, California
What it does: Alpha particles are radioactive elements (ionizing radiation) that are often released from mining waste pollutants and other natural sources. Low levels of alpha particles are not known to be hazardous to our health, but high levels can affect your hair, your gastrointestinal system, and your general health. Anything with radiation typically will cause problems, and you really don't want to be drinking it.