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I have a 1921 home. Guaranteed that the walls have lead paint somewhere beneath, right? All surfaces have been painted at least once if not 2 or 3 times since 2001. The paint is all in good condition - no chipping. To my understanding, this is good encapsulation.

If we had someone do a lead survey with the xray gun, would these surfaces test positive for stuff beneath? Or does the gun ONLY test the surface layer?

I'm curious to know for sure that my surfaces are safe, but due to mandatory reporting/remediation, I don't want to risk testing positive from a sensitive instrument that picks up a signal from a very low risk and covered up surface.

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    No, it's not automatic that old paint has lead. Lead oxides are only capable of making certain colors, and there were alternative pigments even then. – Harper Jun 21 at 5:05
  • Ok sounds reasonable, but white base was one of those colors, right? . As i understand, lead paint was desirable as it went on smoother. So wouldn't they have just added the pigment to white? – Zipper1365 Jun 21 at 22:14
  • Yes, white base is one of the hues, but I was under the impression that it was a low quality white used in cheap paint. That jibes with its social legacy of being found in tenements... – Harper Jun 21 at 22:32
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The short answer is Yes, X-ray lead paint detectors can 'see' lead through top layers of non-lead paint.
But it depends to some degree on the thickness of the layers of paint as well as the type of X-ray detector 'gun' used.

There are 2 types used for lead detection: K X-rays and L X-rays.
The K X-rays have higher energy than the L, which means that they are less attenuated by layers of paint.

So a K X-ray lead detector will be better able to detect lead in lower layers of paint than an L X-ray detector.

If you're concerned that you might get a positive reading from an X-ray lead detector, then you'd probably be better off having a chemical lead test done instead, since this should only be able to find lead in the surface paint layer.

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XRF is testing the exposed surface it is pointed at, it does not "penetrate" to see if there is anything below that. It's not an "X-Ray" as in a medical X-ray that sees through things, it USES an X-ray beam generator to "fluoresce" (make glow) any metals on the surface it is aimed at; think in terms of a fluorescent light making a poster glow with different colors. The gun has a color analyzer that evaluates the colors that are reflected back as to the presence of specific metals, such as lead.

So if your painted surface has multiple layers of "clean" latex paint over the lead paint, the lead is covered up and not exposed to that x-ray beam. However if the latex is flaked off, chipped or worn down, the lead underneath may show up.

Edit; obviously brhans knows more about this than I do, I wasn't aware of there being two types.

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