Arsenic
What is Arsenic?
Arsenic is a naturally occurring semi-metal found in the earth's crust. It enters drinking water mainly by dissolving out of rock and soil into groundwater, and can also come from past industrial and agricultural use. It is most common in private wells and in certain aquifers.
Health effects
Arsenic is a known human carcinogen. Long-term exposure is linked to bladder, lung, and skin cancers, as well as cardiovascular disease and developmental effects. The EPA legal limit is 10 ppb, but EWG's health-based guideline is far lower, around 0.004 ppb, reflecting cancer-risk research suggesting no exposure is truly without risk.
In DC-metro tap water
In our region, arsenic is most associated with the coastal-plain aquifers of Maryland's Eastern Shore and Southern Maryland, and with private wells drilled into arsenic-bearing rock. Because private wells are not covered by municipal testing, checking is the homeowner's responsibility.
How to remove it at the tap
Standard carbon and pitcher filters do not remove arsenic. Reverse osmosis is one of the most effective point-of-use technologies for reducing it, alongside activated alumina and anion exchange. A multi-stage RO system reduces arsenic at the tap regardless of the source.
Where Arsenic shows up locally
Local water reports where this contaminant class is documented in the utility's data or watershed. Open your area to see the specifics and look up your exact zip code.
Frequently asked
Is arsenic in tap water dangerous?
Long-term exposure to arsenic is linked to several cancers and cardiovascular disease, which is why the health-based guideline sits far below the federal legal limit. Reducing exposure matters even at low levels.
Should I test my well for arsenic?
Yes. Private wells are not covered by municipal testing, and arsenic is a common natural well contaminant in certain geologies. Testing is the only way to know your level.
Does reverse osmosis remove arsenic?
Yes. Reverse osmosis is proven to reduce arsenic at the point of use. Carbon and pitcher filters alone do not.
Related guides
The gap between what's legal and what's healthy
A utility can pass every federal test and still deliver water far above the levels health scientists consider safe. Federal legal limits were largely set decades ago; the health-based guidelines below reflect current cancer- and development-risk research.
Federal law allows 2,500× the health guideline
Sources: EPA National Primary Drinking Water Regulations (legal limits) and the EWG Tap Water Database / state public-health goals (health-based guidelines). A reverse osmosis system reduces every contaminant above at the point of use.