_____________________________________________________________________

LEAD Action News Volume 22 Number 4 December 2024 Page 46 of 131

Bone Lead Description and Population Comparison

throughout the body with a particular impact on neurodevelopment.

Even dating back to Roman times, lead’s deleterious effects in the

body were recognized. Lead used to be more ubiquitously used in

paints and leaded gasoline, which led to serious exposures nationwide

with many deaths attributable directly to lead. In the 1990’s, many

studies showed the drastic negative impacts lead was having on the

population including serious effects such as cardiovascular disease

(Navas-Acien, Guallar et al. 2007), neurodevelopmental deficits

(Canfield, Henderson et al. 2003), and death (Weisskopf, Jain et al.

2009). There was a drastic public health intervention to reduce these

Lanphear, Rauch et al. 2018). Thus, communities still experiencing exposures outside of paint and

leaded gasoline had continued health issues associated with lead. For this reason, the Centers for

Disease Control reduced the actionable level of lead exposure from 10 ug/dL to 5 ug/dL (Dignam,

Kaufmann et al. 2019) and most recently the reference level has been further reduced to 3.5 ug/dL.

However, despite these reference values, the scientific community recognizes that there is no safe level

of lead exposure.

Lead can be absorbed through oral ingestion, inhalation, and dermal absorption in the body.

Children have been shown to be much more susceptible to ingested lead than adults with 40-50% of

lead dose orally ingested ultimately being absorbed in the body (United States. Agency for Toxic

Substances and Disease Registry. 2007). This typically leads to high exposures in children from hand-

in which its capabilities as a biomarker of exposure are severely lacking (Rabinowitz 1991). Blood lead

is a biomarker that is highly linked with turnover of red blood cells in the body, as red blood cells are

the primary carriers of lead in blood (O'Flaherty 1998). Thus, in adults, lead in blood has been

repeatedly shown to only reflect recent exposures with a half-life of about 30 days (Nilsson, Attewell et

_____________________________________________________________________

LEAD Action News Volume 22 Number 4 December 2024 Page 47 of 131

al. 1991). This means that if we measured an adult who was severely exposed one month ago, we

would only be able to identify half of the exposure from our lagged measurement. This is further

complicated when we look at children’s biokinetics and blood lead turnover. Previous studies have

shown children have a blood lead half-life of less than one week, so the measurement from blood is

incredibly time sensitive (Specht, Lin et al. 2016, Specht, Weisskopf et al. 2018). This effect introduces

a great deal of uncertainty into a single measurement of blood lead, as, in any given week, the blood

reliant on calcium channels, as well as produce many other ill-effects throughout the body (Bressler

and Goldstein 1991). Lead will also replace calcium in bone. Not only is lead in bone a good marker of

overall exposure, but, since it is directly related to disruption in calcium metabolism, it is a proxy for

lead’s effects in the brain and throughout the body (United States. Agency for Toxic Substances and

Disease Registry. 2007). This is what makes bone such an excellent marker of exposure. Additionally,

since bone changes slowly in the body, studies have shown that lead measured from bone is reflective

of years to decades worth of exposure due to the slow kinetic processes of bone remodeling (Nilsson,

Attewell et al. 1991, Rabinowitz 1991, Barbosa, Tanus-Santos et al. 2005). Thus, a single measurement

of bone lead reflects the lead exposure during a time period of potential exposure from an acute or

chronic source in an individual’s past (i.e., the water crisis). Thus, we can use bone lead to identify lead

sample. Displacement of this electron makes the sample atoms unstable and electrons jump from

outer orbitals to lower orbitals releasing fluorescence, characteristic of the element of each atom. The

fluorescence in the form of secondary X-rays is measured by a radiation spectrometer and the rate of

determination of these secondary rays gives the elemental concentration present in the sample. XRF

has been used for decades to determine the bone lead level of individuals from children to adults in a

variety of different settings (Wielopolski, Ellis et al. 1986, Bleecker, Mcneill et al. 1995, McNeill, Stokes

et al. 2000, Grashow, Spiro et al. 2013, Specht, Lin et al. 2018). Lead can be measured at multiple

energies particular to the XRF device, the K- and L-shell. K and L are reflective of the electron orbitals

in which the measurement arose. K-shell requires the use of a radioisotope source or a high energy

radiation source. L-shell can be used with either radioisotope or x-ray tube sources. Both have been

operate (Hu, Milder et al. 1989). The measurement systems were also incredibly large, requiring

_____________________________________________________________________

LEAD Action News Volume 22 Number 4 December 2024 Page 48 of 131

special high-purity germanium detectors with liquid nitrogen cooling in order to operate properly.

Finally, the measurements were slow with each measurement taking 30 minutes to complete. Thus,

the initial XRF bone lead measurement systems were incredibly large, difficult to obtain, difficult to

operate, and took a significant amount of time for each measurement.

Improvements in low energy (L-shell) x-ray detection led to advancements of low energy XRF

technology (Wielopolski, Ellis et al. 1986, Nie, Sanchez et al. 2011, Specht, Weisskopf et al. 2014).

Previous L-shell measurement systems had similar long measurement times as the K-shell systems in

order to reach detection limits needed for measurement of bone lead. In the early 2000’s, low energy

x-ray detector systems vastly improved in both the maximum counts per second (resolving time) and

energy resolution, which would drastically lower the detection limits and, most importantly, the

XRF devices (Specht, Weisskopf et al. 2014, Zhang et al. 2022). This allows for easy, convenient

measurements of bone lead in field environments and the ability for widespread testing for cumulative

lead exposure.

The current portable XRF measures the same bone lead measurement as has been done

decades prior. This has been verified in theoretical, human, animal, and lab settings using multiple

methodologies for validation (Nie, Sanchez et al. 2011, Specht, Weisskopf et al. 2014, Specht, Lin et al.

known exposure sources from industry. Populations of adults that do not have blood lead levels

typically considered elevated, can still have bone lead levels consistent with that of an elevated level of

exposure. Bone lead in adults from a study in Toronto recently showed a level on average of about 3.4

μg/g bone mineral for those with an average age of 43 (McNeill, Fisher et al. 2017). From the same

study, those with an average age of 29 years had a bone lead measurement of 0.9 μg/g bone mineral

and those with average age of 56.5 years had 6.0 μg/g bone mineral (McNeill, Fisher et al. 2017).

Correcting these levels to values reflective of general population bone lead measurements during the

time of this case would change the values for ages of 29, 43, and 56 years to 0.5, 1.7, and 3.0 μg/g bone

mineral (McNeill, Fisher et al. 2017).

For children, we have a couple of points for comparison. In a group of lead poisoned children

mineral (average ± standard deviation) (McNeill, Fisher et al. 2017). Populations of children that do

not have blood lead levels typically considered elevated, can still have bone lead levels consistent with

that of an elevated level of exposure.

_____________________________________________________________________