These THMs are known carcinogens. But THMs can increase in the distribution pipes, especially if chlorine is added later in the distribution system. Pesticides such as chlordane, used for years for termite control and toxins like PCBs and dioxins. There are MCLs for many of these contaminants, but none are set at zero. Scientists suspect that the combining of these contaminants actually multiplies the harmful effects as new, unknown compounds are formed.
Contaminants come and go through the water supply.
And tests can get very expensive. If you have an older home, you may want to test for lead. Test kits are readily available at hardware stores. You may also want to request a printout from your local water utility to find out what they monitor in the water supply. To find a device that reduces the most number of contaminants, consult the NSF International listing of certified drinking water treatment units, including the contaminants reduced by each model.