DISCUSSION
In a representative US population higher levels of urinary
cadmium were found to be significant predictors of lower
FVC, FEV1, and FEV1/FVC in current and former smokers but not in never smokers. In addition, variability in urine cadmium
levels was found among current smokers. It is not clear
whether this finding is related to the dose of cadmium to
which smokers are exposed or to variability in the uptake and
metabolism of cadmium in smokers. It is possible that the
variability in cadmium levels in smokers is related to the
intensity of smoking—that is, smokers with higher cadmium
levels extract more tar and nicotine out of each cigarette than
smokers with lower levels. It is also possible that the
variability in lung function in smokers is similarly related
to variability in smoking intensity, and that cadmium levels
are just a long term marker of tobacco dose.
Cadmium intake in humans is either through ingestion or
inhalation, with the major source of exposure in never
smokers being ingestion and in smokers being inhalation.27
The differences we observed—with significant decreases
in lung function occurring only in current and former smokers—may be because urinary cadmium levels reflect
lung cadmium better in smokers than in never smokers.28
One cigarette contains about 2 mg cadmium, 2–10% of
which is transferred to primary cigarette smoke.29 Data from
the Massachusetts Benchmark Study showed considerable
variation in the cadmium content of mainstream smoke from
27 brands of tested cigarettes, ranging from 31 to 222 ng/
cigarette with a median level of 137 ng/cigarette.30 Smokers
have previously been shown to have higher blood and urinary
cadmium levels22 31 32 as well as higher lung tissue cadmium
levels.33–35 Urinary cadmium levels have been found to correlate
closely with lung cadmium levels in current and former
smokers.28 Urinary levels of cadmium rise with renal dysfunction36
37 and need to be adjusted accordingly. Adjustment of
urinary cadmium to creatinine reduces variability between
individuals.23 Cadmium levels in the lung and urine remain
raised for many years after smoking cessation Cadmium damages pulmonary cells in vitro, affecting
several levels of function including repair of DNA,38 cellular
enzyme activity and membrane structure,39 and a1-antitrypsin
inhibitory capacity.40 Cadmium has been shown to be
associated with the development of emphysema in occupationally
exposed cohorts,13 although its role in the development
of lung cancer is less clear.41–43
Bjorkman et al found that up to 10% of the variability in
cadmium concentrations was related to genetic factors.44 The
expression and regulation of metallothionein proteins, which
absorb and sequester cadmium and protect against cadmium
toxicity,45 46 may be part of the mechanism of genetic
variation observed in cadmium uptake and metabolism.
This study is subject to some limitations. Because it is cross
sectional in design, we cannot say that higher cadmium levels
led to lower lung function. Smoking status and smoking
histories were self-reported, with the potential for misclassification
of these critically important measures. Occupational
or non-vocational exposures to cadmium (such as alloy making,
jewellery making, etc) were not specifically asked about.
In conclusion, we found that urinary cadmium levels,
which reflect the total body burden of cadmium, are inversely
associated with lung function in current and former smokers
but not in never smokers. Future research should examine
whether lung cadmium levels are predictors of lung function
decline, the extent to which variation in cadmium levels
among smokers reflects cadmium dose versus cadmium
uptake and metabolism, and whether increased levels of
cadmium in the lung are amenable to intervention.