What are xenobiotics, and why some of them linger

A woman in her forties is tired, gaining weight she cannot explain and sleeping badly. Her bloods are unremarkable. She mentions, near the end, that she has repainted and refitted two flats in the past year and did most of the sanding herself.

That sentence is worth more than another panel. It is also the sentence nobody asks for, because the question that produces it is rarely part of a consultation.

What is a xenobiotic?

A xenobiotic is any compound that is foreign to the body’s own chemistry: not a nutrient, not something you make. Medicines are xenobiotics. So are pesticide residues, solvents, plasticisers, flame retardants, the metals in old paint, and the smoke from anything burning.

The body has one broad system for handling all of them, and it does not care where a compound came from. Your liver converts fat-friendly compounds into water-friendly ones so bile and the kidneys can post them out, and that machinery treats a painkiller and a paint solvent with the same chemistry.

Which is why what happens next depends less on the compound than on where it can hide.

Why do some compounds linger for years?

A water-friendly compound is cleared quickly. A fat-friendly one has somewhere to sit, and body fat is the largest store available. Compounds that resist being broken down and prefer fat, including several older pesticides and industrial compounds, are measurable in people decades after exposure stopped.

That store is not sealed. When fat is broken down quickly, in rapid weight loss, illness or after bariatric surgery, what was held in it is released back into circulation. Blood levels of persistent compounds rise during weight loss, which is measured and expected.

This is why the history matters more than most tests, and why the questions have to be specific.

What does a useful exposure history ask?

Ask about work first, and about every job held, not the current one. Painting, printing, welding, salons, dry cleaning, agriculture, dentistry, laboratories, the fire service and construction all carry recognised exposures.

Then ask about the building: its age, whether anything was renovated or sanded, whether there is damp, and what the water supply is. Then the hobbies, which are where unprotected exposure concentrates: furniture restoration, soldering, model making, ceramics, shooting ranges, and anything involving strippers or solvents in a closed room.

Ask what changed when. An exposure history is worth taking when the symptoms have a beginning, because the useful question is what else began around the same time.

At some point someone will ask whether a test would settle it.

Which tests are worth doing?

For a few specific exposures, testing is well established and genuinely useful. Blood lead is the standard for lead. Blood or urine mercury is used for mercury, with the form and the timing mattering. Occupational medicine has validated markers for solvents, and those are the tests to ask for when the history points somewhere specific.

The broad “toxic burden” panels sold directly to the public are a different matter. They measure many compounds at once, report against ranges built from small reference groups, and cannot tell you when the exposure happened or whether it explains any symptom.

Which leaves the useful part: what actually reduces the load.

What reduces exposure and helps clearance?

Removing the source does more than anything else, and it is often within reach: ventilation while sanding or painting, a respirator rather than a dust mask, wet cutting instead of dry, changing out of work clothes before sitting down at home, and filtered water where supply is the issue.

Clearance itself runs on ordinary inputs. Protein supplies the building blocks for the second stage of conversion, fibre carries compounds out in bile rather than letting them be reabsorbed, and normal bowel and kidney function do the rest. Lose weight steadily rather than crash-dieting, particularly if exposure has been long-standing.

Turning this into a consultation that takes four minutes rather than forty, and knowing which answers change the plan, is the subject of the Deep Dive on taking an exposure history.