[As far as I remember a Canadian NGO had found lead and cadmium in vaccines. Instead of checking the vaccines, the Canadian Govt cut off funding to the NGO. Arsenic is another metal suspected to be in vaccines.- Jagannath]
Low-level exposure to cadmium and lead may increase risk of hearing loss, researchers said.
Analysis of 1999-2004 data from the National Health and Nutrition Examination Survey (NHANES) indicated that individuals in the highest quintile of blood cadmium levels were significantly more likely to show hearing loss of more than 25 dB at speech frequencies, relative to those in the lowest quintile (adjusted odds ratio 1.74, 95% CI 1.12 to 2.70), reported Sung Kyun Park, ScD, MPH, of the University of Michigan in Ann Arbor, Mich., and colleagues.
Although corresponding results for blood lead levels did not reach statistical significance, the researchers did find that hearing thresholds increased by 5.41% (95% CI 2.12% to 8.81%) for each doubling in lead concentrations in blood. A quantitatively similar effect was seen for blood cadmium levels.
Park and colleagues reported the findings online in Environmental Health Perspectives. The results reflected adjustments for demographic variables and risk factors including occupational, recreational, and firearm-related noise exposure, as well as certain medical conditions.
"The present study supports the hypothesis that environmental cadmium and lead exposures at levels currently observed in the U.S. may increase the risk of hearing loss, the third leading chronic condition experienced by adults ages 65 and older," they wrote. "Our findings support efforts to reduce environmental cadmium and lead exposures to effectively prevent or delay hearing loss in the general population."
Animal studies had indicated that heavy metals such as cadmium and lead can induce hearing loss, but evidence for such a relationship in humans was scant, Park and colleagues explained.
The only previous epidemiological study to examine cadmium exposure and hearing loss was conducted in adolescents, they pointed out.
To look for such relationships in the general population, the researchers turned to the NHANES program, which collects blood samples from participants. Among the tests performed on these samples is analysis of lead and cadmium levels.
Additionally, from 1999 to 2004, a subset of about 5,000 NHANES participants underwent hearing tests. Park and colleagues used data from 3,698 participants, after excluding those with unilateral hearing loss or with information missing on important potential confounders such as exposure to firearm noises.
The weighted mean age in the sample was 42 and the pure-tone average hearing threshold was 12.8 dB (SD 0.2). A total of 441 participants had an average threshold of at least 25 dB above normal, defined as hearing loss, in one or both ears.
Compared with other participants, Park and colleagues wrote, those with hearing loss at this level had significantly higher age-adjusted levels of blood cadmium (0.46 versus 0.40 mcg/dL) and lead (1.72 versus 1.52 mcg/dL) (P value not stated).
When participants were grouped into quintiles of blood cadmium and lead levels, there was a clear relationship between these levels and the degree of hearing impairment.
With the lowest quintiles for each metal as the reference, the fully adjusted statistical models showed increases in pure-tone average hearing thresholds of 1.68%, 6.69%, and 13.78% for the third, fourth, and fifth quintiles of blood cadmium, respectively (P=0.005 for trend).
Similarly, the third, fourth, and fifth quintiles of blood lead had increases in the hearing thresholds of 6.51%, 10.22%, and 18.63%, respectively (P<0.001 for trend).
Although the findings could not prove causality, Park and colleagues noted that the previous animal studies had shown that these metals have a variety of toxic effects within the auditory system, such as induction of apoptosis in inner ear receptor cells. The outward effects were increases in hearing thresholds like those seen in the NHANES participants.
Limitations to the analysis include the data's cross-sectional nature and the lack of correlation between one-time blood lead and cadmium measurements and long-term exposures, the researchers noted.
The study was funded by the National Institute of Environmental Health Sciences.
Study authors declared they had no relevant financial interests.