One of the favorite scare words that alties like to use is "chemicals." It's a very vague term, just the way the dishonest ones like it. Strictly speaking, a chemical is anything made of atoms, ions, and molecules:
Edward Elric: Water, 35 liters. Carbon, 20 kilograms. Ammonia, 4 liters. Lime, 1.5 kilograms. Phosphorous, 800 grams. Salt, 250 grams. Saltpeter, 100 grams. Sulfur, 80 grams. Fluorine, 7.5, iron, 5, silicon, 3 grams, and trace amounts of 15 other elements.I don't know about Ed's accuracy (though bear in mind he lives in a world where chemical bonds are easy to overcome with chalk circles), but the point should be clear: Even your own body is made of chemicals. Because of that, there's no way for us to avoid them as a whole.
Rose: What's that?
Edward Elric: It's all the ingredients of the average adult human body, right down to the last specks of protein in your eyelashes...
-Fullmetal Alchemist, Episode 1: To Challenge the Sun.
The tricky part comes from avoiding the bad chemicals. First, it doesn't matter if the chemical is natural or artificial: Dihydrogen monoxide is dihydrogen monoxide, whether it's made in a plant or in a lab. Second, any chemical, even the ones we need to live can be poisonous in large amounts: Drinking too much water can lead to hyponatremia as it flushes out our electrolytes. Oxygen in large enough amounts can kill: Scuba divers learned that the hard way back when they used pure oxygen in their tanks. As we skeptics like to say, the dose makes the poison.
One thing I like to do when faced with a scaremonger abusing the term is to replace the word "chemical" with "stuff." It takes the sting out of it all, doesn't it? If you're in an argument with such a person, request that they specify which chemicals they're talking about, as well as the doses at which they become harmful.