Why Measure Lead in Bone

Home Up

 

 

Why Measure Lead in Bone?

Contents

The Health Effects of Lead

Lead in Blood

Blood-lead is not known for the general population

Lead in Bone

Possible Mobilization of lead from bone

X-Ray Fluorescence measures long-term lead exposure

 

The Health Effects of Lead

The harmful health effects of lead have been known for thousands of years, but observations of injurious effects at low levels of lead have been the subject of increasing concern in the past few years. Recent research has pointed to possible dangers faced by specific populations who risk lead exposure from mobilization of their body lead stores, e.g., children, pregnant women and osteoporotics. Lead toxicity is reported to be a major public health problem in the United States today. The general population is exposed to lead in their environment. This lead can come from several sources, e.g., house paint, water and soil. Although lead has been banned from house paint, older housing stock still contains lead paint which can contaminate household dust. Lead was removed from American gasoline in the early 1980s, and lead levels in children has fallen considerably. Yet this previous use has resulted in soil contamination which still exists. Lead continues in use in many plumbing fittings. Many areas still receive their water supply through lead pipes. All of these factors can lead to an elevated total lead consumption.

Environmental exposure to lead is not the only source of lead-related health effects. Many industrial workers in the United States have potential occupational exposure to lead, and lead poisoning is still seen at occupational health clinics.

Clinical lead poisoning in itself does not define the extent of lead-related health problems. Recent research has shown that increased lead exposure, even at legally permissible levels, can lead to harmful, though sub-clinical, effects. Some of the earliest symptoms of the ailment are non-specific, such as fatigue and muscle pain and are frequently ascribed to factors other than lead poisoning. Other effects include changes in kidney function, inhibited central nervous system function and reduced nerve conduction velocity, the latter having been demonstrated in lead workers who showed no symptoms. This means that more adults may be affected by both environmental and occupational lead exposure than can be estimated from the numbers who present at clinics.

Lead in Blood

At the present time, exposure to lead is most commonly monitored by measuring blood-lead levels. The criteria for lead poisoning and lead toxicity are based on blood-lead as a standard. However, the biological half-life of lead in blood is approximately 36 days. It is therefore an indicator only of recent lead exposure. Blood-lead reflects chronic exposure only if exposure is constant and the measurements were constant and well documented. Deleterious health effects of lead resulting from long-term lead exposure will only be correlated with current blood-lead levels if lead exposure has been relatively constant over a long period of time, up to the time of sampling.

Measuring lead in blood has methodological drawbacks and limits on physiological interpretation. Methodologically, measuring lead in blood frequently requires a venous sample and sending blood to a laboratory for analysis. In most states, the delay between sampling and analytic results can be 6-8 weeks. This delay obviously impedes efficient public health prevention, since in the absence of immediate feedback no decision can be made on risk reduction at the time of initial screening or clinic visit. Often, it is difficult to locate the persons who were sampled after this delay, and, of course, exposures may continue in the interval. Physiologically, the measurement of lead in blood is not a direct assessment of target organ dose, since the red cell is not a critical target for lead toxicity. Kinetically, blood is not a good analog for critical targets, such as soft tissue, because of the relatively short half-life of lead in blood as compared to target organs or bone.

Blood-lead is not known for the general population

Long-term lead exposure is of primary health concern but can rarely be ascertained from blood-lead records. No one in the general population has an adequate blood-lead measurement history. However, 109Cd K X-Ray Fluorescence (XRF) bone-lead measurements allow the direct measurement of long-term lead exposure.

At the present time, little is known about the range of chronic environmental exposures in the general population. Further research is required for the full implications of chronic lead exposure to be thoroughly understood. However, 109Cd K XRF bone-lead measurements have the potential to enhance our understanding of the effects of low-level lead exposure and consequently to determine whether the current intervention criteria, which are based on blood-lead levels, afford adequate protection against the effects of lead. Bone-lead measurements may also provide an additional screening technique in the identification of high-risk populations.

Lead in Bone

Lead is predominantly stored in the human body in calcified tissues; 90-95% of the total lead burden is contained within bone in non-occupationally exposed adults. The total lead content of bone is reported to be up to 200 mg in 60-70 year old men, less in women. The turnover rate of lead in cortical and trabecular bone is slow; quantitative estimates of the half-life vary, but there is a consensus that it is of the order of years or even decades. Therefore, through childhood and most of adult life, lead exposure from both environmental and occupational sources results in an increased lead concentration within the bone matrix. A measure of bone-lead content thus reflects integrated or cumulative, and thus long-term or chronic, lead exposure and provides a useful surrogate indicator of the cumulative dose of lead presented over time to the target organs of lead.

In vivo bone-lead measurements may therefore clarify the risks associated with lead exposure in two ways. Health effects which are associated with chronic lead exposure may be identified by their correlation with bone-lead level, and bone-lead measurements may ultimately allow the identification of subjects at risk from mobilization of their body lead stores and allow appropriate intervention strategies to be devised.

Possible mobilization of lead from bone

Under conditions where bone physiology is undergoing a period of change, such as during pregnancy, aging and osteoporosis, it would appear that lead can be released from the bone mineral matrix, increasing blood-lead levels and constituting a further source of lead exposure. It would seem likely that the level of this endogenous exposure would be dependent on bone-lead burden.

K X-Ray Fluorescence measures long-term lead exposure

Several large in vivo studies have confirmed that the integrated 109Cd K XRF bone-lead measurement is a measure of long-term lead exposure. Bone-lead measurements have been performed on more than 800 occupationally exposed workers in England, Sweden and Finland in several studies. The occupationally exposed groups were studied as extensive blood-lead records were available on these subjects; in some cases records extended as far back as 1950. It was found in all the individual studies that bone-lead level of all bone sites including the tibia, calcaneus and sternum, correlated with the "Cumulative Blood-lead Index" (CBLI). The CBLI is an integrated time-weighted average blood-lead level and thus corresponds to total lead exposure. The studies reported that 109Cd K XRF bone-lead measurements could therefore be considered to be a measure of cumulative lead exposure.

Evidence therefore exists that the 109Cd K XRF method can provide an accurate measurement of bone-lead level, to well within the currently available levels of precision, and further that this measurement of lead in bone can be considered to be a measure of long-term lead exposure.

The radiation dose and consequent risk arising from a K XRF bone-lead measurement are very small for all age groups, including children.