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|Year : 2009 | Volume
| Issue : 44 | Page : 141--144
Heavy metals and noise exposure: Health effects
Audiology Department, Royal Surrey County Hospital, Guildford, Surrey, United Kingdom
Audiology Department, Royal Surrey County Hospital, Guildford, Surrey
Heavy metals are chemical elements with a specific gravity that is atleast five times that of water which is 1 at 4�C. The commonly encountered heavy metals are lead,mercury, cadmium, and arsenic. Lead levels in children continue to be a health hazard as the current limit of 10ug/dL is considered too high with the WHO estimate of 40% of children having blood levels greater than 5ug/dL. Some authors have suggested a new limit should be set at 2ug/dL. There are substantial differences in the literature regarding the effects of lead on hearing as assessed by pure tone audiometry. Mercury causes hearing loss and neurological dysfunction in Humans and animals. Methyl mercury is considered a more toxic compound to mercuric chloride. Cadmium causes a range of health effects from lung cancer, kidney damage to hearing loss. Dose dependent effects on hearing loss have been shown in rats. Combined effect with noise exposure has been shown to be more pronounced. Arsenic is released into the environment through the smelting process of copper, zinc, and lead. It is usually found in the water supply. Hearing impairments have been noted in the low and high frequencies in conjunction with balance disturbance.
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Prasher D. Heavy metals and noise exposure: Health effects.Noise Health 2009;11:141-144
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Prasher D. Heavy metals and noise exposure: Health effects. Noise Health [serial online] 2009 [cited 2023 May 31 ];11:141-144
Available from: https://www.noiseandhealth.org/text.asp?2009/11/44/141/53358
"Heavy metals" are chemical elements with a specific gravity that is at least five times the specific gravity of water, which is 1 at 4�C. The specific gravity may be considered as a measure of density of a given amount of solid substance, when it is compared to an equal amount of water. For example some heavy metal specific gravities are: Arsenic, 5.7; cadmium, 8.7; iron, 7.9; lead, 11.3; and mercury, 13.5.  As these heavy metals are not metabolized by the body, they accumulate in the soft tissues or in the bones causing toxic effects. Heavy metals may enter the human body through food, water, air, or absorption through the skin, when they come in contact with humans in residential and occupational settings as well as from the general environment. The commonly encountered toxic heavy metals are lead, mercury, cadmium, and arsenic.
Every year the industry produces about 2.5 million tons of lead throughout the world, most of which is used for batteries. The remainder is used for cable coverings, plumbing, ammunition, and fuel additives. Other uses include paint pigments, PVC plastics, x-ray shielding, crystal glass manufacture, pencils, glazing ceramics, canned foods with lead-solder joints, ethnic foods, herbal remedies, dietary supplements, lead emissions from fossil fuels, metal smelting, and pesticides. Box 1- [SUPPORTING:1] lists the health effects of lead.
According to the World Health Organization's (WHO) World Health Report, following control measures, lead levels have been steadily declining in industrialized countries, but yet, at least 5% of the children still have elevated blood lead levels, with even higher rates in children of poorer households. In many developing countries, where leaded gasoline is still used, lead can present a threat to more than half the population of children. Rapidly increasing traffic loads have the potential to further increase blood lead levels.
The CDC (Center for Disease Control) set a limit of 10 ug/ dL for children, in USA, in 1991, based on the studies of cognitive deficits. Of late it has been considered that there is sufficient and compelling evidence to lower the limit values as toxic effects have been observed below 10 ug/dL. Gilbert and Weiss  argue for a 2 ug/dL limit.
The WHO estimates that worldwide, approximately 120 million people have lead levels of 5-10 ug/dL, with similar numbers above 10 ug/dL, and 40% of the children have blood lead levels above 5 ug/dL. Overall, 97% of the affected children live in developing regions.
Patel et al. ,  reported that 50% of the children tested had PbB greater than 10 ug/dL, in India. In a recent study, Jain and Hu  reported that children under 3 years had blood lead levels between 5 and 20 ug/dL. In their study, a low standard of living correlated with a 32% increase in blood lead levels. Children in the ninety-fifth percentile for their weight/height, compared to below 5%, had higher blood lead levels by 31%.
Lead affects practically all body systems. The most toxic exposures occur at chronic low levels and can result in reduction in intelligence quotient (IQ), increased blood pressure, and a range of behavioral and developmental effects. The range and extent of adverse health effects have been appreciated relatively recently. Furthermore, lead is now understood to be toxic, especially to children, at levels previously thought to be safe. In more severe cases of poisoning, adverse health effects include gastrointestinal symptoms, anemia, neurological damage, and renal impairment. Other adverse effects, such as reduction in IQ levels, behavioral disorders, or renal dysfunction, can be discerned only through special examinations. These analyses estimate that lead results in about 234 000 (0.4%) deaths and 12.9 million (0.9%) DALYs (Disability Adjusted Life Years).
Lead and Hearing Loss
There are contradictory findings on hearing loss from lead exposure. According to Farahat et al.  and Forst et al. ,  lead exposure induces hearing loss, but others ,,, have indicated that this is not the case. Some studies have shown a relationship between blood lead levels and hearing loss, ,, but Counter et al. ,  were unable to substantiate this in their study. It has also been suggested , that lead exposure results in hearing loss, which in turn may be responsible for developmental learning disabilities. Buchanan studies, ,, have shown that Andean children between the ages of 5 and 14 years, with blood lead levels of 33-118 ug/dL (which is 3-12 times higher than the US limit), had normal hearing thresholds and distortion product otoacoustic emissions (DPOAE) and observed no correlation with blood lead levels. Osman et al ,  reported delayed wave I of the auditory brainstem response in children exposed to lead. It is clear that there are substantial differences in the current literature on the effect of lead exposure on the auditory sensitivity indicated by pure tone audiometry.
Lead and noise: Effect on hearing
There are very few studies exploring the effects of combined lead and noise exposure. Wu et al. ,  examined 220 workers exposed to lead (56.9 ug/dL) and noise (86 dB Laeq) in a lead-battery manufacturing factory. Multivariate analysis showed a significant correlation between high long-term lead exposure (duration of employment and ambient lead concentration), with the hearing threshold at 4 kHz, but no correlation was observed with short-term exposure. The effect due to lead and noise were not considered separately. No enhancement of hearing thresholds were reported with lead and noise combined. Long-term lead exposure in ceramic glazing workers, with a mean blood lead level of 45 ug/dL, has been shown to have sensorineural hearing loss in 60% men and 20% women. The raised thresholds between 2 and 8 kHz may be due to the combined exposure to lead and noise. Absolute brainstem response latencies were prolonged, but the inter-wave intervals were normal, indicating a conductive element to the hearing loss. Counter and Buchanan  suggested that environmental exposure must be considered an important factor in adults who were occupationally exposed to lead.
Lead and noise: Human growth
Lead exposure has an impact on Human growth and development,  and noise stress is also considered to be implicated in the reduced prenatal growth.
The use of mercury is wide-spread as shown in Box 2 - [SUPPORTING:2]. The health effects also cover many neurological and psychological symptoms as shown in Box 3 - [SUPPORTING:3].
Mercury: Hearing loss
Mercury intoxication causes hearing loss in humans and animals. In 1953, a severe neurological disorder was recognized among persons living in the vicinity of Minimata, Japan, where mercury-containing effluent flowing from a chemical plant into the local bay contaminated shell fish. After the incident, deterioration in hearing and deafness were reported among other neurological symptoms, in the local people. Findings consistent with Minimata disease have been reported in other instances of accidental mercury poisoning in Japan and Iraq. Early stages of intoxication may result in cochlear lesions, whereas, hearing loss in the late stages of intoxication may result from neurological damage.
Methyl mercury is considered more toxic than mercuric chloride. Hearing loss due to Methyl mercury has been reported,  while wave III of the auditory brainstem response has been shown to be delayed and used as a biomarker for prenatal MeHg toxicity from contaminated seafood. Dimethyl mercury, Methyl mercury, and Mercuric Sulfide have all been shown to affect auditory brainstem potentials. ,,,
Cadmium is used in nickel-cadmium batteries, PVC plastics, and paint pigments. It can be found in soils because insecticides, fungicides, sludge, and commercial fertilizers that use cadmium are used in agriculture. Cadmium may also be found in reservoirs containing shell fish. Cigarettes also contain cadmium. Lesser-known sources of exposure are dental alloys, electroplating, motor oil, and exhaust. Inhalation accounts for 15-50% of absorption,and 2-7% of ingested cadmium is absorbed in the gastrointestinal system.
The health effects range from occupational lung cancer, kidney damage from tubular dysfunction to chronic renal failure, skeletal damage, and hearing dysfunction.
Dose-dependent effect on hearing has been shown in rats by Agirdir and Ozcagalar, in 2002. Increased blood and renal cortical cadmium levels were associated with high cadmium accumulation in ear ossicles and labyrinth in rats exposed to cadmium. The changes in auditory brainstem responses and otoacoustic emissions in two-month old male rats exposed to drinking water containing 5 and 15 ppm, for 30 days, showed that cadmium-induced nephrotoxicity was associated with signs of defective hearing at a concentration of 15 ppm, but at 5 ppm it caused hearing loss without affecting the kidney function. The mean latency of ABR wave I, which indicated the function of the cochlea, was 1.335 � 0.31 ms in the control group and significantly prolonged to 1.641 � 0.052 and 1.74 � 0.88 ms in rats subjected to 5 and 15 ppm, respectively. Nonsignificant changes in wave III and V latencies were accepted as evidence of an unaltered function of the other parts of the auditory system. These results suggest that hair cells are more sensitive to cadmium than kidney tubule cells and that the cochlear component of hearing is more vulnerable to cadmium toxicity than other parts of the auditory system. 
Cadmium has a dose-dependent deleterious effect on the auditory system in rats. Preventive effect of a zinc-enriched diet on cadmium-induced hearing loss in rats was investigated by Agirdir and Ozcaglar. , The control rats were fed normal rat food and tap water, while the cadmium group was subjected to 15 ppm cadmium-containing water as CdCl 2 . A third group received 15 ppm CdCl 2 and food enriched with 200 ppm zinc as ZnSO 4 for 30 days. Hearing function was measured by using the auditory brainstem response and distortion product otoacoustic emission. Blood cadmium increased from 1.87 � 1.69 to 6.08 � 2.62 �g/dL and the elevated cadmium contents of the ear ossicles and kidney cortex were associated with a decreased glomerular filtration rate in rats subjected to high cadmium. A zinc-enriched diet obviously reduced cadmium accumulation in the kidney and prevented nephrotoxicity. Cadmium-induced ototoxicity seemed to be partially zinc preventable and addition of zinc to the diet without altering the cadmium content in the ear ossicles might help to prevent cadmium- induced hearing loss.
Cadmium and noise: Fetal malformations
Mice were exposed to a wide octave-band of noise at 100 dB(C) for 6 hours on day 7 of pregnancy and Cadmium sulfate at 1 or 2 mg/kg was intraperitoneally injected. On day 18 of pregnancy, the fetuses were examined for external and skeletal malformations. In the groups exposed to continuous noise for six hours, the total percentage of malformed fetuses was significantly higher than that in the control group. Although combined treatment with cadmium and noise resulted in an increase in the total percentage of malformed fetuses compared to the same dose of cadmium alone, the interactions between cadmium and noise showed no synergistic effect on the teratogenicity. The magnitude of teratogenicity due to noise was much weaker than that of cadmium, and was therefore easily masked by that of cadmium in the statistical tests of the significance of differences.
Cadmium and noise: Hearing
DeAbreu and Suzuki  examined the effect of cadmium fumes and noise exposure and showed that hearing loss at 4 kHz and 6 kHz was more severely affected with combined exposure.
Arsenic is released into the environment by the smelting process of copper, zinc, and lead, as well as by the manufacturing of chemicals and glass. Arsine gas is a common by-product produced by the manufacturing of pesticides that contains arsenic. Arsenic may also be found in water supplies worldwide, leading to exposure in shellfish, cod, and haddock. Other sources are paints, rat poisoning, fungicides, and wood preservatives.
Target organs are: Blood, kidneys, and the central nervous, digestive, and skin systems.
Considerable variability among individual arsenic values in the hair makes group examination a necessity. Hair, urine, and blood samples taken from groups of 10-year-old boys, each having 20 to 25 individuals, residing in a region polluted by arsenic and hearing changes were analyzed in a group of 56 10-year old children residing near a power plant burning local coal of high arsenic content. In the case of air conduction, significant hearing losses were found at frequencies of 125, 250, and 8000 Hz. The changes were particularly marked in the low-frequency region. The high statistical significance of hearing impairments found, pointed to a very low probability of there being only an "accidental" finding. The possibility of toxic damage to the ear could yet not be excluded according to Bencko and Symon. 
There are inconsistent findings on lead-induced hearing loss. It is also not clear whether lead and noise combined exacerbate hearing loss, but they do affect human fetal growth. Mercury affects hearing, with central conduction time delay (ABR I-V, III-V), but cochlear function may be unaffected. Cadmium causes dose-dependent loss of hearing in rats, and a delay in only Wave I of the auditory brainstem response implies cochlear dysfunction. A zinc-enriched diet reduces the ototoxic effect of cadmium. Cadmium and noise show a synergistic effect at 4 kHz and 6 kHz, and on human fetal malformations. Arsenic produces a low- and high-frequency loss with balance disturbance.
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