Occurrence Of Mercury Research Articles

Overview of mercury occurrence in Bulgarian groundwater: Preliminary data

Mercury and its compounds are listed as hazardous substances and prohibited by Bulgarian regulations for discharge in groundwater. Regional studies on the natural distribution of mercury in groundwater are scarce but in recent decades a significant amount of data has been accumulated in the National Environmental Monitoring System. We analysed groundwater samples from two different regions and the results showed Hg concentrations similar to published data and not significantly different from values measured in nearby surface waters, with no patterns found by host rock type. Correspondence between the published data and the results presented here indicate typical background Hg concentrations between 0.1 µg/L and 0.3 µg/L for most regions in Bulgaria. Additional studies with a larger geographical scope are needed for a more detailed characterization of the natural variations in background mercury contents in groundwater in Bulgaria.

Findings on the heavy metal content in herbal drugs and essential oils – An updated database evaluation

In order to guarantee the safety of patients and consumers, the European legal framework restricts the content of heavy metals and further elements in plant material used for the production of medicinal and food products. This data evaluation comprises findings on seven elements between 2016 and 2021 which were assessed against the requirements of the European Pharmacopoeia (Ph.Eur.) and the ICH Q3D guideline on elemental impurities. !5.328 herbal drug samples from 427 products and 537 essential oil samples from 25 products were included in this evaluation which demonstrates that the Ph. Eur. limits set for cadmium, lead and mercury are considered appropriate. Samples from organic production revealed a lower occurrence of cadmium, lead and mercury as compared to samples from conventional production. With regard to the ICH Q3D requirements, none of the herbal drug samples exceeded the respective limit for cobalt, nickel and vanadium, while for arsenic higher values were observed in individual cases. A comparison of the findings with a former period of evaluation showed an increase in positive findings and also a few more exceedances of the Ph.Eur. limits. For essential oils, it can be confirmed that independent from the production process, the probability of heavy metal contamination is low.

Content, association and potential modes of occurrence of mercury in diverse biomass types and their ecological significance

Content, association and potential modes of occurrence of mercury in diverse biomass types and their ecological significance

A Pilot Study on Bioaccumulation and Tissue Distribution of Mercury in Barn Swallow (Hirundo rustica).

Although extensive research has been carried out on the occurrence of mercury (Hg) in biota, bioaccumulation and tissue distribution of Hg in songbirds have not been well characterized. In the present study, Hg was investigated in insects and barn swallows (Hirundo rustica) to explore the bioaccumulation characteristics of Hg. Hg in swallow feathers and tissues including muscle, liver, and bone was investigated to determine the tissue distribution of Hg. The concentrations of Hg were 1.39 ± 1.01 μg/g, 0.33 ± 0.09 μg/g, 0.47 ± 0.10 μg/g, and 0.23 ± 0.09 μg/g in feather, muscle, liver, and bone samples, respectively. The trophic magnification factor of Hg in swallows and insects was higher than 1. However, the Hg concentrations in swallow feathers were not significantly correlated with stable isotope values of carbon or nitrogen, which implies the complex food sources and exposure processes of Hg for swallows. Feathers had significantly higher concentrations of Hg than liver, muscle, and bone samples (p < 0.01 for all comparisons). Feather, muscle, bone, and other organs had fractions of 64.4 ± 11.9%, 6.07 ± 2.06%, 20.0 ± 8.19%, and 9.56 ± 2.96% in total body burden of Hg in swallows. Hg in feathers contributed more than half of Hg in the whole body for most swallow individuals. Swallows may efficiently eliminate Hg by molting, and the excretion flux of Hg and other contaminants via molting deserves more investigation.

Mercury occurrence and speciation in sediments from hard coal mining in Czechia

This study examines Hg distribution in stream sediments impacted by hard coal mining in the Upper Silesian Coal Basin (USCB), Czechia. By means of a comparative analysis, geological samples and samples from stream sediments were used to carry out a comprehensive assessment of the effects of anthropogenic activities on Hg distribution and speciation. Total Hg (THg), total organic and inorganic carbons (TOC and TIC), and total sulphur (TS) were measured in the samples to reveal a potential relationship. In addition, THg and TS species were discussed in order to elucidate their mobility pattern in the environment. The results have shown that there are no correlations between THg, TS, and TOC indicating overlapping Hg sources attributed to industrial processes. Geological samples, particularly coal and associated sedimentary rocks, contained lower Hg concentrations compared to a variety of stream sediments. The main Hg species identified in the samples was a stable β-HgS, which decreases its mobility in the riverine environment. It follows that Hg enrichment and speciation is linked to industrial processes, which are the main origin/cause for Hg enrichment and transformation. Minor proportions of HgO in some samples show Hg oxidation upon diagenesis, while HgCl2 is attributed to the chemical loads from the former coking plant.

Distributions of total mercury and methylmercury and regulating factors in lake water and surface sediment in the cold-arid Wuliangsuhai Lake region.

This study aimed to understand the occurrence of mercury in the water environment of typical cold and arid lakes and the regulating environmental factors. Water and surface sediment samples were collected from July to August, 2022 in the Wuliangsuhai Lake region for the analysis of total mercury (THg) and total methylmercury (TMeHg). Lake water THg and TMeHg ranged between 19.20 ~ 668.10 and 0.10 ~ 11.40ng/L, respectively, exceeding China's environmental quality standards and contents of other lakes and reservoirs in China and other areas. Surface sediments showed lower mean THg and TMeHg of 261.85 and 0.18μg/kg, respectively, with the former significantly exceeding the background value of Inner Mongolia and unpolluted natural lakes but lower than those of lakes affected by human factors, such as aquaculture. Sediments showed relatively low methylation and TMeHg (0.01-0.21%) concentrations. Correlation analysis identified salinity, total dissolved solids, conductivity, and redox potential as important factors affecting mercury speciation in water, whereas those in surface sediments were organic matter, pH, and total iron content. This study conducted preliminary research on the different species of Hg in Wuliangsuhai Lake water environment, which can provide scientific evidence for the specific treatment of Hg pollution in agriculture, or industry and other related fields. Our results suggest that upstream and downstream regulatory agencies should strengthen the regulation of agricultural and industrial production, moderately reduce human activities, and reduce the use of mercury-rich substances such as pesticides.

Occurrence and risk assessment of total mercury, methylmercury and other selected trace metals in the surface sediments of the Cochin Estuary, Southwest Coast of India

This study assesses the ecological risk and impact of anthropogenic activities in the Cochin estuary – a Ramsar site – by trace metal indices in surface sediment collected from thirteen sampling stations in three seasons. The trace metals analyzed are THg, MeHg, Pb, Cu, Cd, Zn, Ni, Co, Mn, Cr, Fe, and Al in the surface sediments and their spatio-temporal variation is recorded. Pollution indices (i.e., Enrichment factor, Index of geoaccumulation, sediment quality guidelines, pollution load index, and potential ecological risk index) were applied to ascertain the sediment quality. The distribution pattern of Cr, Zn, and Cd revealed higher concentrations in the northern arm of the estuary and near the industrial regions indicating non-geogenic inputs. The concentration of THg and MeHg exhibited spatial and temporal variations and were higher than the background values. The results showed that higher levels of trace metals were found in the northern part of the Cochin Estuary, which could be attributed to the weak flow and significant input of industrial effluents.

Occurrence of mercury in polychaete species (Annelida) and their associated sediments from an important Southern Atlantic Ocean Bay

In the present study, the Hg levels in freshly collected polychaete species and their associated sediments were evaluated from Todos os Santos Bay, Northeastern Brazil. Additionally, it was also measured the Hg distribution along the annelids' body parts (anterior region, abdomen, and posterior region). Total Hg concentration was as follows: 13.6–144 μg kg−1 (Scolelepis chilensis, deposit feeder), 8.2–122 μg kg−1 (Laeonereis acuta, deposit feeder), 95–612 μg kg−1 (Armandia agilis, deposit feeder); 96.6–206 μg kg−1 (Lumbrineris sp., carnivorous), 75.3–112 μg kg−1 (Goniada echinulate, carnivorous), and 115–198 μg kg−1 (Branchiomma sp., suspension feeder). In turn, Hg levels in sediments ranged from 2.77 ± 0.24 to 6.38 ± 0.15 μg kg−1. Hg concentrations found in polychaete soft tissues were higher than those found in the sediments. Specific habits, feeding behavior, trophic level, and stressful environmental conditions are the main factors affecting the Hg levels in the species studied. The bioaccumulation factor (BAF), contamination factor (CF), and ecological risk assessment (Er) were calculated. The BAF values were higher than 1 for all studied species, indicating Hg bioaccumulation in the annelids, but the low levels of CF e Er showed those Hg levels present low to no significant ecological risk for the biota and benthonic organisms living in the sediments. Finally, no statistically significant difference was observed for Hg concentration levels along the polychaete body parts.

Mercury occurrence in drinking water resources of Ganga Alluvial Plain, northern India

Mercury occurrence in drinking water resources of Ganga Alluvial Plain, northern India

Effects of Mercury Concentration on the Health and Safety of Oil and Gas Workers

Introduction: The processing of crude oil is characterized by numerous hazards, which have significant health, safety, and environmental impacts on neighboring communities. The occurrence of mercury and its toxic derivatives is considered one of the many negative impacts of oil and gas operations. However, there is limited research on mercury and its negative effects on workers in the Gulf Cooperation Council (GCC) region. This study examines the occurrence, exposure, and symptoms of mercury on the health and safety of oil and gas workers in Oman. It also explores the acute/chronic effects of mercury poisoning on the maintenance and inspection workers, who are more prone to the adverse effects of mercury poisoning during oil and gas operations. Methods: The two-pronged approach of literature review and survey questionnaire was used to deduce the effects of mercury exposure and poisoning using 68 respondents with 1 – 6 years of working experience in the sector. Results: Based on the questionnaire response rate of 72%, the results showed that over 90% of participants had experienced symptoms of elemental, organic, and methyl mercury poisoning. The common symptoms experienced are headaches, insomnia, weakness, hearing impairment, visual and sensory abilities. However, the respondents who experienced methyl mercury (MeHg) symptoms are due to consuming seafood and cigarettes, whereas elemental and organic mercury symptoms are due to oil and gas operations. Conclusion: The findings highlight the need for robust health and safety measures to effectively detect, monitor and eliminate mercury compounds responsible for poisoning maintenance and inspection workers.

Remediation alternatives for an abandoned gold mine contaminated with mercury in Brazil / Alternativas para descontaminação de mina de ouro abandonada contaminada com mercúrio no Brasil

Heavy metal contamination of soils has generated two major problems, namely, loss of land value and health risk for people near contaminated sites. Studies on soil remediation are typically conducted on a pilot scale under very controlled conditions. This is a misleading approach, once the complexity of the actual biogeochemical soil conditions may causes inefficiency in full scale real-world application. In addition, environmental impacts of remediation practices themselves are often ignored, which may make them unfeasible. In 2002, an occurrence of mercury in an abandoned old gold mine was detected in a rural area of Descoberto, Brazil. In 2014, the State Public Prosecutor required from the responsible State Environment Agency, a remediation project to the area. This study compares alternatives for remediation of the contaminated area. Also a historical approach on mercury amalgamation in gold mining in the region, in order to estimate the amount of mercury remaining in the site. The geochemical characterization of the contaminated area was performed to describe the contamination process. The results were compared with the remediation proposal advocated by the Brazilian Ministry Public, and some alternatives for decontamination of the area were studied. The electrokinetic method has displayed the lowest overall unit cost, between US$ 120.00/t and US$ 260.00/t. The residual Hg concentration after four cycles of the electrokinetic technique was estimated at 1.28 mg (Hg)/kg (soil), slightly higher than the intervention value for agricultural soil, which is 1.20 mg (Hg)/kg (soil).

Occurrence of Mercury Along with Different Body Parts of Nine Polychaete Species (Annelida) and Their Associated Sediments from an Important Southern Atlantic Ocean Bay

Occurrence of Mercury Along with Different Body Parts of Nine Polychaete Species (Annelida) and Their Associated Sediments from an Important Southern Atlantic Ocean Bay

Distribution, sources and forms of mercury occurrence in the street dust of Mezhdurechensk (Kemerovo Region)

Abstract. The aim of this study was to investigate mercury concentration and forms of its occurrence in the dust component of the atmospheric air (street dust) to assess the geo-ecological condition of the area where coal is mined, processed and transported. The mean mercury concentration of 29 street dust samples collected along a uniform network throughout the city was 36.4 4.7 ng/g, with a minimum (4.6 ng/g) and a maximum (104.6 ng/g) value, respectively. This value was much lower than the mean concentration in soil samples taken previously at the same sites (58.4 ng/g) but higher than the mean concentration in snow solids (29.8 ng/g), which was likely due to precipitation that washed out mercury from the street dust and to the wind blowing. The method of successive chemical extraction revealed a weakly bonded water-soluble form, inorganic poorly sorbed mercury compounds, an organic form of mercury, and mercury associated with minerals. The proportion of each fraction accounted for on average 10.94%, 10.32%, 44.76%, and 33.97%, respectively. Water-soluble and acid-soluble mercury forms were less common. The bulk of mercury had metal-organic and sulfide forms. The study of mercury distribution based on particle size indicated that the mercury concentration increased as the diameter of the grains of dust material decreased. The fine fraction with a diameter of 20 to 50 m contained the bulk of mercury, which stayed in suspension form for a long time before settling on the surface.

Contamination of the Dzheltulak-1 river basin under alluvial gold mining (the Amur region)

The purpose of the introduced research is to study the environmental impact of placer gold mining. The object of the study is natural and natural-man-made geosystems of the Dzheltulak-1 river located in the Amur region. The content of the main cations and microelements in water samples was determined by atomic emission and mass spectral methods. The content of micro- and rock-forming elements in bottom sediments and soils was determined by X-ray fluorescence using XRF-1800 X-ray spectrometer (Shimadzu, Japan). The results of the conducted studies indicate that the contents of iron, manganese, copper, aluminum, vanadium, molybdenum, mercury and ammonium ions exceed commercial fishery standards in a significant part of the surface water samples. It is found out that settlement ponds negatively affect the water quality in the river Dzheltulak-1 downstream due to the dam drainage. The highest concentrations of dissolved forms of mercury were noted in the water of the settlement ponds. The gross content of arsenic in the samples of bottom sediments and soils exceeds sanitary and hygienic standards by 2.7–14.5 times. The distributions of mercury as the most dangerous pollutant among the biogenic components of geochemical landscape were worked out in detail. Mercury contamination of soils was classified according to the forms of mercury occurrence: free, physically sorbed, chemisorbed, sulfide and isomorphic. It has been shown that the free form is characteristic of fresh mercury contamination due to the use of prohibited technological schemes for gold-bearing sands separation by the amalgamation method.

Relating mercury occurrence in soil gases at establishments hosting children to historical mercury-using activities in Paris, France

Volatile pollutants from former industrial sites can degrade the buildings' indoor air quality that were built after the industrial activities. Since 2010, environmental assessments have been conducted in French establishments hosting sensitive populations identified as being on or near potentially contaminated former industrial sites. These projects are based on historical studies traditionally carried out as part of managing contaminated sites, to determine which substances should be analyzed. They pinpoint former activities likely to have stored or used pollutants. We show that the historical information collected is not effective in targeting sites with increased probability of mercury being present in soil gases. Environmental history has demonstrated the existence of large-scale artisanal contamination, both prior to and concomitant with the industrial era. Classic historical studies would not take into account artisanal activities, which are less documented than industrial activities. We carried out additional research for three schools located in three different Parisian districts. Although information on activities which could have emitted mercury was relatively imprecise (in terms of location, type and duration of activities) and uncertainties exist about the completeness of the archival documents available, our investigations identified several mercury-using activities that had not been identified during the classic historical study. However, we have shown that the number of activities identified does not provide information on how mercury has affected soil gas. Consequently, although a more extensive historical research improves knowledge about the presence of potential mercury-using activities, our study shows that a systematic analysis of mercury as part of the assessment of establishments hosting sensitive populations remains relevant. This approach should be applied to other cities around the world.

Occurrence of total mercury and methylmercury in rice: Exposure and health implications in Nepal

Emerging studies have indicated that the consumption of rice could be the major methylmercury (MeHg) contributor to human mercury (Hg) exposure. Nonetheless, few studies are available on Hg in rice around the world, especially in countries with heavy rice diet. In this study, total Hg (THg) and MeHg levels in rice samples (n = 172) across Nepal were first investigated. The geometric mean THg was 7.05 ± 7.71 µg/kg with a range of 0.622 µg/kg to 158 µg/kg, and the maximum THg level was up to 791% of the Chinese National Standard Limit for THg in rice (20 µg/kg). The geometric mean MeHg was 0.820 ± 0.660 µg/kg with a range of 0.189 µg/kg to 8.59 µg/kg. Overall, the mean MeHg exposure (0.00445 ± 0.00477 µg/kg bw/day) and inorganic Hg (IHg) exposure (0.0360 ± 0.0739 µg/kg bw/day) were lower than the reference dose (RfD) of 0.1 µg/kg bw/day for MeHg and the provisional tolerable weekly intake (PTWI) of 0.571 µg/kg bw/day for IHg, respectively. Concerning different groups of vulnerable populations, the highest MeHg exposure (0.126 µg/kg bw/day) and IHg exposure (1.57 µg/kg bw/day) of preschoolers (37–50 months old) were approximately 126% of the RfD for MeHg and 275% of the PTWI for IHg. When the pregnant mothers eat the rice without awareness of the Hg content in rice, the mean and highest intelligence quotients (IQs) losses were 9554 and 118659 points, respectively, and the corresponding economic costs due to IQ loss could be 15.1 million USD and 188 million USD in Nepal. The results of rice THg and MeHg levels and corresponding exposure in populations highlighted the occurrence of rice THg and MeHg pollution issues in Nepal. More efforts should be made to protect younger groups in Nepal from high rice Hg exposure. CapsuleOwing to the high rice consumption rates relative to body mass, preschoolers (37–50 months) may meet the 126% reference dose (0.1 µg/kg bw/day) for MeHg and 275% provisional tolerable weekly intake (0.571 µg/kg bw/day) for IHg exposure in Nepal.

Correlation of Mercury Occurrence with Age, Elemental Composition, and Life History in Sea-Run Food Fish from the Canadian Arctic Archipelago’s Lower Northwest Passage

As mercury emissions continue and climate-mediated permafrost thaw increases the burden of this contaminant in northern waters, Inuit from a Northwest passage community in the Canadian Arctic Archipelago pressed for an assessment of their subsistence catches. Sea-run salmonids (n = 537) comprising Arctic char (Salvelinus alpinus), lake trout (S. namaycush), lake whitefish (Coregonus clupeaformis), and cisco (C. autumnalis, C. sardinella) were analyzed for muscle mercury. Methylmercury is a neurotoxin and bioaccumulated with fish age, but other factors including selenium and other elements, diet and trophic level as assessed by stable isotopes of nitrogen (δ15N) and carbon (δ13C), as well as growth rate, condition, and geographic origin, also contributed depending on the species, even though all the fish shared a similar anadromous or sea-run life history. Although mean mercury concentrations for most of the species were ~0.09 µg·g−1 wet weight (ww), below the levels described in several jurisdictions for subsistence fisheries (0.2 µg·g−1 ww), 70% of lake trout were above this guideline (0.35 µg·g−1 ww), and 19% exceeded the 2.5-fold higher levels for commercial sale. We thus urge the development of consumption advisories for lake trout for the protection of pregnant women and young children and that additionally, periodic community-based monitoring be initiated.

Biochemical, physiological (haematological, oxygen-consumption rate) and behavioural effects of mercury exposures on the freshwater snail, Bellamya bengalensis

The widespread occurrence of Mercury (Hg) and its derivatives in the aquatic environment and risks to the health of local populations has necessitated investigations into its toxic effects on sessile species. The toxicity of Mercury was observed sequentially from 96 h acute exposure regime (behavioural endpoints) to chronic durations (haematological and biochemical toxicity endpoints) in Bellamya bengalensis. Time-dependent lethal endpoints for acute toxicity (LC50) of mercury i.e., 24,48,72 and 96 h were estimated as 0.94, 0.88, 0.69 and 0.40 mg/l respectively. Threshold effect values i.e., LOEC (Lowest Observed Effect Concentration), NOEC (No Observed Effect Concentration) and MATC (Maximum Acceptable Toxicant Concentration) at 96 h were found to be 0.10, 0.05, 0.039 mg/l respectively. The study of oxygen consumption rate and behavioural changes during acute toxicity and haematological and biochemical responses during chronic toxicity to sublethal concentrations (10% and 20% of 96 h LC50) of mercury to the snail were also conducted. The organisms showed initial elevation at 24 h but later gradual decrease in oxygen consumption rate with the increase of concentration of mercury and time of exposure. For behavioural studies, variable test concentrations from 0.00 to 1.00 mg/l were used for 24, 48, 72 and 96 h. The crawling activity and clumping tendency decreased with the progress of time at all treatment periods and stopped ultimately at 96 h of exposure from 0.7 mg/l onwards whereas touch reflex was not observed at 96 h exposure at all treatments except at 0.09 mg/l. In haemocyte count, no significant variation was observed among control values between various exposure periods (p > 0.05) though variations were observed in sub-lethal concentrations versus control at all treatment duration (7, 14, 21, 28d, p < 0.05). In biochemical response study, the protein content in hepatopancreas of the snails treated at sublethal concentrations of mercury (10% and 20% of 96 h LC50) reduced significantly versus control after 21d of exposure (p < 0.05). In gonads, the protein content of the treated snails significantly reduced at all treatment concentrations versus control at all exposure times (p < 0.05). Based on the safe levels indicated above, the concentration of 0.01 to 0.04 ppm of mercury can be considered safe for Bellamya bengalensis and any less-hardy aquatic species. These responses elicited by our molluscan model will not only help in biomonitoring of environmental mercury contamination in water bodies but will also provide support to ecological health and risk assessment.

May a comprehensive mineralogical study of a jackstone calculus and some other human bladder stones unveil health and environmental implications?

This paper represents the first result of an active collaboration between the University of Sannio and the San Pio Hospital (Benevento, Italy), started in the 2018, that aims to a detailed mineralogical investigation of urinary stones of patients from Campania region. Herein, selected human bladder stones have been deeply characterized for clinical purposes and environmental biomonitoring, focusing on the importance to evaluate the concentration and distribution of undesired trace elements by means of microscopic techniques in the place of conventional wet chemical analyses. A rare bladder stone with a sea-urchin appearance, known as jackstone calculus, were also investigated (along with bladder stones made of uric acid and brushite) by means a comprehensive analytical approach, including Synchrotron X-ray Diffraction and Simultaneous Thermal Analyses. Main clinical assumptions were inferred according to the morpho-constitutional classification of bladder stones and information about patient's medical history and lifestyle. In most of the analyzed uroliths, undesired trace elements such as copper, cadmium, lead, chromium, mercury and arsenic have been detected and generally attributable to environmental pollution or contaminated food. Simultaneous occurrence of selenium and mercury should denote a methylmercury detoxification process, probably leading to the formation of a very rare HgSe compound known as tiemannite.

Obstetricians’ engagement for successful recruitment of participants in a randomized controlled trial promoting healthful fish consumption during pregnancy

BACKGROUND AND AIM: Human biomonitoring trials depend on the voluntary participation of citizens, who donate biological samples and personal information related to the investigated chemical exposures. Pregnant women are vulnerable to exposure to mercury, a potent developmental neurotoxicant. In Europe, the primary source of exposure to mercury relates to fish consumption. At the same time, fish consumption during pregnancy provides essential nutrients for optimal fetal brain and eye development. The benefits can outweigh the risks if suitable dietary advice is provided during pregnancy. METHODS: Cyprus, a small coastal Southeastern European country, used national data of fish consumption and mercury occurrence in fish to develop the first ever recommendations for Cypriot pregnant women. These are tested in the European “HBM4EU-mom” randomized control trial (10/2020 – 12/2021, Horizon2020 Programme Contract No.733032 ) with the involvement of 130 women of 18-45 years in a healthy singleton pregnancy, willing to eat fish. 15 trained obstetricians nation-wide recruit women under their care and take their hair sample during routine pregnancy appointments. RESULTS:Within one month, 78 pregnant women, corresponding to 60% of the recruitment target, were successfully recruited and provided hair samples. Virtually all women invited to join the study, consent to participation. Some women provide their informed consent at the time of invitation, while others review the information for participants at home and return the completed certificate of informed consent at the next scheduled meeting. Based on current data, it is predicted that the recruitment target will be reached within two months total. CONCLUSIONS:Recruitment of pregnant women and sampling by obstetricians is successful because it builds on the trust that the pregnant women feel towards their health care provider. It also overcomes obstacles due to the COVID-19 pandemic, creates educational opportunities for the doctors and facilitates the communication of personal results to the participants via their health care provider. KEYWORDS: Community-engaged research, Policy, Chemical exposures, Heavy metals, Exposure assessment, Pregnancy outcomes