Arsenic is a well known human carcinogen that occurs naturally in groundwater. Public water systems are regulated by the US EPA to never exceed 10 parts per billion (ppb) of arsenic concentration, but health research demonstrates that no level of arsenic is safe to drink.

In fact, EPA proposed 5 PPB (parts per billion or ug/L) as a standard in 1996. Levels this low have shown statistically significant impacts on IQ among children, but economic requirements for setting maximum contaminant levels have kept the official threshold higher than levels harmful to health.

If you want to remove arsenic from your tap water, there are a few important things you need to know.

The total arsenic concentration is generally comprised of two common oxidation states (or forms): arsenic III (aka arsenite) and arsenic V (aka arsenate).

In drinking water, we are usually worried about Arsenic V. If you drink water with Arsenic V, however, it is quickly converted to Arsenic III–which is more toxic and bioreactive inside your body.

The source of your water makes a difference to the amount of arsenic III versus arsenic V in your water. This matters for choosing appropriate treatment options.

If your water is coming from a community or public water treatment system and that treatment system uses chlorine for disinfection, then you can generally assume that most arsenic in your water is in arsenic V form.

If your drinking water comes from a groundwater well and there is no chlorination (or other oxidative step) installed as existing treatment, then you can’t be sure what fraction of your total arsenic is comprised by arsenic III or arsenic V.

A type of water quality test called Arsenic Speciation can help determine if your total arsenic is mostly arsenic III or arsenic V.

Reverse osmosis water treatment technology can reliably remove arsenic V, but does not reliably remove arsenic III.

If your water’s arsenic is all or mostly arsenic V then you can reliably use reverse osmosis technology to remove arsenic from your drinking water.

If your water’s arsenic is mostly comprised of arsenic III then you will either need a special treatment technology OR you will need an additional treatment step to oxidize arsenic III to arsenic V before using reverse osmosis for treatment.

Reverse osmosis is not the ONLY method of arsenic removal. For example, ion exchange and filtration-based treatment options also exist..

Whole home water treatment for arsenic removal is NOT necessary unless you regularly drink water from many of the taps in the house.

You can safely choose to treat only the water you ingest or use for drinking by installing a point of use filter at your main water source in the home or building (like under your kitchen sink). This can save thousands of dollars, because according to public health research, arsenic does not show an appreciable impact on health via showers, tubs, or brushing teeth (unless concentrations are enormous, >1,000 parts per billion).

It is important to note that no treatment technology can ever truly remove a contaminant 100%. Water treatment with reverse osmosis can remove arsenic as much as 98%, however. Most professionals and public health research suggests trying to reduce arsenic intake by drinking water to less than 3 parts per billion.