Arsenic is a famous poison. It is still used to kill insects and other pests but many of those compounds are being phased out. The other big use is as a wood preservative. Chromated copper arsenate (CCA) treatment gives construction lumber that pale green color you see mostly in outdoor applications. They used to make picnic tables and playgrounds from it but the EPA put a stop to that.

Arsenic can kill you by repeated exposures to very low doses. Groundwater (and thus well water) can often have high concentrations of arsenic. It is found in sulfide minerals which are widely dispersed throughout the American West. Mine tailings are a particularly troublesome source of arsenic but many aquifers are contaminated by natural processes from natural sources.

If you drink well water you should test it for arsenic. You should test it anyway, just to be on the safe side, of course. Municipalities are required to test their water and report its chemistry to the users. I have just about every one of the annual City of Yreka water test results. Water that has too many minerals or other such problems can usually be treated and made safe.

Arsenic is used in the semiconductor industry. Two spaces to the left on the chart is Gallium (Ga, #31). You can see that arsenic is one column to the right of the carbon family (the column with C, Si, Ge, Sn, Pb, Fl) and that gallium is one column to the left. Silicon, like carbon, has four valence electrons. What we call the Information Age could be called the Silicon Age. Elements to the left of silicon have three valence electrons and elements to the right have five valence electrons. This means that both arsenic and gallium work as dopants or impurities that are deliberately added to silicon to change its electronic properties. Arsenic and gallium also combine to form gallium arsenide (GaAs) which is used in many integrated circuits, diodes, and solar cells.

Arsenic is added to the lead used in car batteries. Imagine how many millions of car batteries there are. And I mean the old-fashioned 12-volt starter-type batteries, not the new-fangled EV batteries. Just think about how many cars there are and how many of those have had multiple batteries. That’s a lot of lead and thus a lot of arsenic.

Arsenic in the global food supply is a matter of international concern. Contaminated irrigation water can cause arsenic accumulation in cereal grains and particularly in rice and rice products. Arsenic also gets into the air from the burning of fossil fuels.