There is growing interest in the health effects of manganese exposure. It has long been recognized that, in occupational settings, significant exposure to manganese (Mn) can occur among workers involved in ferroalloy production, iron, or steel foundries, and welding.[1] Mn in the forms of dust and fume is released in these processes, and enter the body primarily via inhalation and lead to adverse health effects. Short-term assessment of exposure to Mn among workers is generally conducted by measurement of airborne Mn from the worker’s breathing zone, which assesses the estimated external dose for the duration of measurement. The general population is also exposed to manganese, although generally at lower levels than workers, largely through water, food, or air.[1]

In epidemiologic studies, biomarkers of exposure are useful to verify subject-specific exposures and as an index of internal dose. Blood manganese, urinary manganese, and less frequently nail and hair manganese, have been suggested as biomarkers of exposure.[2] Blood Mn and urinary Mn may reflect exposure over a short and recent period of time (hours to days), while nails and hair are thought to integrate exposure over longer periods of time and thus represent exposures over several months or longer.[3-6] These biological markers can provide important information on the relationship between exposure, the biologically effective dose of an agent at a susceptible site, and the eventual development of clinically-apparent outcome.[7-8]

The utility of blood and urine Mn as biomarkers of Mn exposure is questionable based on the findings from several studies.[2, 5, 9-10] A recent study among occupationally exposed adults [5] found a complex and limited relation between exposure (as measured by total air Mn level and cumulative exposure indices) and blood Mn levels that may depend upon exposure attributes and the latency of blood measurements relative to exposure. Plasma and urine Mn appeared even less useful as exposure biomarkers. In general, studies have found no or weak relations between blood or urine Mn and exposure, which could be explained by the high variability and short half-life of Mn in blood and urine.

Toenails and hair have been proposed as exposure biomarkers for longer-term cumulative exposure assessment. A number of previous studies have demonstrated that Mn can accumulate in toenails, fingernails, and hair [4, 6, 9, 11-12], and that Mn accumulation is greater in these matrices among high exposure groups than lower exposure groups.[9, 12] These studies, however, have not examined the individual level correlation between external Mn exposure and hair or toenail Mn. A validated biomarker for cumulative exposure to Mn could be of tremendous use in studies of health effects of Mn exposure.

To determine how well toenail Mn reflects cumulative exposures in different exposure windows, we examined the correlation between Mn cumulative exposure indices and toenail Mn concentration among welders at a union welding school. We focused on toenails because hair is thought to be more subject to accumulation of metals due to external contamination, reflecting concentrations out of proportion to the body’s actual internal exposure.[4] We studied welders because the determination of external exposures is difficult among the general population whereas specific exposure histories are more easily taken among welders, particularly those with a more limited range of welding jobs like those at the welding school. We also examined the correlation between toenail, blood, and urine Mn concentrations, and the change in blood and urine Mn following Mn exposure over a work shift.

From the personal exposure monitoring over their work shifts, the concentrations of airborne Mn from welding ranged from 2.02 to 137.41 μg/m³ (median = 12.89 μg/m³). Mn-CEI (median) ranged from 0 – 6.88 (0.52) mg/m³-hr for the 7-9 month window, 0 – 5.05 (0.44) mg/m³-hr for the 10-12 month window, 0 – 21.88 (1.23) mg/m³-hr for the 1-6 month window, 0 – 10.09 (1.15) mg/m³-hr for the 7-12 month window, and 0 – 24.14 (3.41) mg/m³-hr for the past 12 months. Blood Mn (n= 27) collected at the beginning of the day ranged from 15.29 to 28.61 ng/ml (mean = 18.88 ng/ml) and beginning of the day urinary Mn (n = 27) ranged from 0.45 to 3.02 ng/ml (mean = 1.50 ng/ml). Collected toenail samples (n=49) had a skewed distribution ranging from 0.05 to 10.41 μg/g with a median of 0.80 μg/g (25^th-75^th percentiles: 0.45-1.49). The distribution of toenail Mn by different characteristics and Mn-CEI for the different exposure windows, are presented in table 1. Toenail Mn tended to increase with age, was higher in whites, and increased with the Mn-CEIs for the exposure windows between 7 and 12 months prior to the toenail clipping date. However, toenail Mn did not appear to be higher with increased dietary Mn intake. There was little relation between either blood Mn or urinary Mn and any of the variables (data not shown).

The Spearman correlations between toenail Mn and past Mn-CEI, adjusted for age and dietary Mn intake, were significant for the 3 Mn-CEI windows encompassing months 7-12 before the toenail clipping date (7^th-9^th months, 10^th-12^th months, and 7^th-12^th months), ranging from 0.32 to 0.35 (table 2). The unadjusted correlations were similar but slightly weaker. Pearson correlation analyses on square root transformed toenail Mn and Mn-CEI also yielded similar coefficients. There was little correlation between toenail Mn and any of the other Mn-CEI exposure windows, nor either pre-work shift blood Mn or urinary Mn. In contrast, the adjusted correlation between pre-work shift blood Mn and urinary Mn was reasonably high (r=0.33) and close to significant (p = 0.10). There was little correlation between post-work shift blood and urine (r=0.06; p=0.78), nor between the change in each (r=0.03; p=0.88).

There was no correlation between air Mn exposure over a work shift (mean duration = 5.83 hours; sd = 1.02 hours) and either post-shift blood Mn concentration or change in blood Mn concentration over the work shift (table 3). Air Mn exposure over a work shift was slightly more correlated with post-work shift urinary Mn and change in urinary Mn over the work shift, although these did not reach statistical significance. Pre- and post- work shift concentrations of both blood Mn and urinary Mn, however, were significantly correlated. Pre- work shift blood Mn and urinary Mn were also each correlated with the change in those measures, although inversely and only statistically significantly for change in blood Mn (table 3).

Our study demonstrates that toenail Mn, averaged over clippings from all 10 toes, is correlated with Mn-CEI windows encompassing months 7-12 before the toenail clipping date, which is the exposure window it would be expected to reflect based on toenail growth rates. One of the first estimates reported a toenail growth rate of approximately 0.050 mm per day [19], which would suggest that clippings should reflect exposures, on average, 8-9 months before the clipping date. A recent study [20] found slightly faster nail growth rates, and suggested that the great toenails grow faster than other toenails—at the estimated rate of 2.09 mm/month. Since it is much longer than other toenails, great toenails (average length of 2 cm) may represent exposures more than 10 months before clipping.[20] The average exposure window reflected would be 1-2 months earlier than this if nails from all 10 toes were used. These are consistent with the findings from our study that the correlation between toenail Mn and Mn-CEI was strongest for exposure windows in the 7-12 months before clipping. This is also consistent with a previous study of toenails as markers of selenium intake.[25] Toenail Se concentration was unaffected by dietary intake in the 3 months prior to nail clipping and appeared to provide a time-integrated measure of intake over a period of 26-52 weeks (~6.5-12 months) before nail clipping, which is essentially the same exposure window as ours.

A few studies have suggested that the human toenail could be used as a biomarker of exposure to other elements such as arsenic [24] and selenium.[25-26] However, we are aware of only two studies that looked at toenails as biomarkers of Mn exposure. One study compared toenail Mn to whole blood Mn and fecal Mn collected at the same time among exposed workers.[6] No assessment of individual level external exposures, however, were made in this study. They observed poor correlations between the different biomarkers and concluded that toenails might not be a good biomarker for Mn. Another study in a general population sample compared toenail Mn among multivitamin users to matched controls and found that toenail Mn was not significantly higher among Mn supplement users.[4] This is consistent with our finding that toenail Mn did not tend to increase with dietary Mn, although we did observe slightly stronger correlations with Mn-CEIs after adjusting for age and dietary Mn. Unlike other elements (e.g., As) [24], Mn from food intake may not make a significant contribution to internal dose represented by toenail Mn. It is also possible, however, that increases in toenail Mn from dietary sources could have been masked by a relatively larger amount of occupational exposure to Mn by inhalation.

It was not unexpected that blood Mn and urinary Mn were not well correlated with toenail Mn, nor any of the Mn-CEI given the relatively short half-life (several days) of Mn in blood and urine.[7, 27] A number of previous studies suggested that blood Mn and urinary Mn may be used to discriminate groups of exposed and non-exposed subjects, but not differences in exposure on an individual basis.[2, 5, 9-10] Morning blood and urine samples were reasonably well correlated with each other, suggesting that the exposure windows they reflect are similar. Because of the shorter time frames of blood Mn and urinary Mn as exposure biomarkers, we were interested in how well these biomarkers reflected Mn exposure across a work shift. However, changes in both blood Mn and urinary Mn over a work shift were not well correlated with air Mn exposure over that time. Intriguingly, there was a strong significant inverse correlation between pre-work shift blood Mn and change in blood Mn over the work shift. This suggests that there may be homeostatic pressures that limit the rise in blood Mn such that those who have a high blood Mn concentration before the work shift do not increase blood Mn concentration as much as those whose blood Mn is lower before the work shift. There was a similar although less strong and significant trend for urinary Mn. Small sample size for blood and urine may limit our ability to observe such relationship.

A strength of our study is that the population was recruited from a welding school, which meant that the range of their welding exposures to Mn over the prior year was more limited than in many other settings and we were able to take air measurements during most of the types of welding tasks they performed over the past year. These factors should have helped increase the accuracy of our Mn-CEI. Nonetheless, Mn-CEI created on the basis of retrospective work histories likely have some error that would reduce the correlation between toenail Mn and true Mn exposure for a given window. Thus, the correlations we found may still underestimate correlations with actual air Mn exposures.

A broader implication of our results relate to the use of toenails as a biomarker of Mn exposure in non-occupational settings. In our occupational study setting air Mn exposures are likely far more variable than in the general population because the exposures depend heavily on job or training-related exposure and these can vary substantially from month to month (welders often have periods of intense work activity followed by periods of no work activity and these can vary month to month). As a result air Mn exposure in months 1-6 prior to toenail clipping are less correlated with air Mn exposure 7-12 months prior to clipping than they might be in a general population sample. Thus, the utility of toenail Mn as an exposure metric in an occupational setting is likely restricted to a period 7-12 months prior to the clipping date. However, to the extent that external exposures are more consistent, as might be expected in a general population setting, toenail Mn may be a better biomarker for more general cumulative exposure. Furthermore, toenails have important practical advantages over other samples such as blood and urine in that they are easier to collect (e.g. they can be collected by mail with no special mailing requirements) and store.

Hair and fingernail tissues are similar to toenail and have also been explored to some extent as biomarkers of Mn exposures, although they are generally thought to be more subject to external contamination because they are more frequently open to the external environment than toenails. This may explain in part why the few studies that have explored these tissues as biomarkers of Mn exposure suffered from a relatively great variation as well as from analytical problems.[9, 11]

Our study is, to our knowledge, the first to assess the relation between external exposure to manganese and toenail Mn concentration on an individual basis. Among this population of boilermaker welders, neither blood nor urine was reflective of cumulative exposure in the prior year and changes in Mn concentration in these media over a welding work shift did not correlate well with Mn exposure over that time. Toenail Mn, however, was well correlated with exposures 7-12 months earlier. While we conducted this study among occupationally exposed workers in order to better assess the external exposures, the results may be applicable to the general population, where toenail Mn may well be reflective of a more extended exposure window since external exposures in the ambient settings are likely more consistent than in occupational settings.