Arsenic (As)

The zones above describe totals, which is the trap. A reading of 300 µg/L looks frightening against a flag set at 100, yet if it was drawn the morning after a sushi dinner, the bulk of it may be the organic form the body shrugs off. The same 300 in someone who ate no seafood and drinks from a private well is a different conversation entirely. The number is identical; the meaning is not.

A genuinely high inorganic arsenic, the part that matters, points to an exposure rather than to anything your body made. The greatest source worldwide, according to the WHO, is contaminated groundwater used for drinking, cooking, and irrigating crops, which is why rice and well water come up so often. Inorganic arsenic is a confirmed carcinogen the WHO links to skin, bladder, and lung cancers, and the same agency sets a provisional limit of 10 µg/L for arsenic in drinking water (a water-quality standard, not a blood or urine reference value).

The catch is that a total result cannot tell you any of this by itself. It can only tell you the sum.

This is where speciation earns its place. The ATSDR recommends speciated testing whenever a total comes back elevated, precisely to split the toxic inorganic species from the non-toxic organic seafood forms. Without it, a high total is an unsorted guest list. With it, the burglar (if there is one) finally gets named. The WHO frames the same split cleanly: inorganic arsenic compounds, such as those in water, are highly toxic, while organic compounds, such as those in seafood, are far less harmful.

If your speciated inorganic fraction is what is elevated, the conversation moves to the source. A high total driven by arsenobetaine after a seafood weekend usually resolves itself once the diet column is read.

There is no meaningful "low" arsenic to chase. It is not a nutrient, and the goal is less of it, not more, so a result near zero is the expectation rather than a deficiency. The trickier case is a normal result that still misses something real. Because arsenic leaves the body quickly, a normal number does not always rule out a past exposure.

The ATSDR notes that urinary arsenic falls rapidly in the first 24 to 48 hours after exposure, and blood clears even faster, which is why blood is a poor screen for anything but the most recent contact. A sample drawn a week after the exposure can read reassuringly low while the exposure was genuine. This is the quiet limitation behind every single arsenic value: it captures a narrow window, and a missed window reads as normal.

Arsenic rarely travels alone on a lab order. It usually arrives as part of a heavy metals panel alongside lead and mercury, each a different metal with its own behavior and its own clearance quirks. Many labs also normalize urinary metals to creatinine so that a dilute or concentrated sample does not skew the reading, which is a different kind of correction than the total-versus-speciated split but solves a related problem: making one number actually mean what it claims to.

Two practical reminders close the loop. First, the unit on a urine arsenic result is usually µg/L, while the older blood reference sits in µg/dL, so a stray "1" is not always the same "1." Second, because a total bundles the harmless seafood form in with the toxic one, the speciated inorganic fraction is the figure that actually settles the question. For why a flagged value is an invitation to look closer rather than a conclusion, the guide to reading flagged values is the place to start, and because the total only means something once it is read against its speciated breakdown, our guide to comparing one result against another covers how a paired reading turns an alarming sum into a meaningful one.

The short version: a high total arsenic is a question, not an answer. The seafood you ate is usually the loudest voice in that number, and it takes speciation to find out whether anything else was in the room.