Section III – Risk Assessment:
4 Human Exposure Analysis
4.1 Introduction
To assess the potential human health risk from mercury to the ‘average’ New Zealander, an estimate of the total amount of mercury that an individual is exposed to via all the different exposure pathways (e.g. ingestion, inhalation) is required. In this assessment, dermal exposure (i.e. though skin contact) has been ignored as it is considered to be a relatively unimportant exposure pathway.
The second step in the exposure assessment is comparing the exposure estimate with a health standard for mercury (Section III-2.5) to establish if the exposure is acceptable.
Mercury exposure is idiosyncratic, as individual exposure may vary widely depending on individual circumstances, e.g. the amount and type of fish consumed and the number of amalgam dental fillings. In this human exposure assessment, average exposure estimates have been used to determine the concentrations of mercury that the ‘average’ person is exposed to. It is not possible to make individual estimates without a detailed study of the person to account for all intake routes.
The concept of the average person is somewhat of a convenient fiction. Although most people will fall in the middle zone, in reality there will be a distribution of mercury intake figures for individuals ranging from very low to very high, some of whom may exceed the recommended guideline values in Section III-2.5.
4.2 General Population
4.2.1 Dietary Exposure
The level of mercury in foods is variable and partly reflects the concentration of mercury in the soil and water in the area where the food was cultivated or captured. In the 2003/04 New Zealand Total Diet Survey (NZTDS) most of the food consumed in New Zealand did not have detectable concentrations of mercury (generally less than 2 µg/kg) with the exception of fish, which had elevated concentrations of mercury (Vannoort and Thomson, 2005).
The concentration of mercury in most oceanic fish is generally around or less than 0.15 mg/kg, however large and long-lived predatory fish species (shark, tuna, lemon fish) typically have mercury levels in the range of 0.20-1.5 mg/kg (Vannoort and Thompson, 2005). Battered fish measured in the 2003/04 NZTDS had concentrations of up to 0.85 mg/kg (Vannoort and Thompson, 2005). Organic mercury, particularly methylmercury, is the predominant form of mercury in fish, thought to comprise roughly 80% of mercury in fish (Tchounwou et al, 2003).
The WHO set a new PTWI of 1.6 µg/kg bw/week for methylmercury in 2003 (WHO, 2004), while the PTWI for total mercury is 5 µg/kg bw/week or 350 µg/person/week for a 70 kg adult (WHO, 1972). The NZFSA estimates average daily intake of mercury by adults from food sources ranges from approximately 0.60 to 0.74 µg/kg bw/week (Table III-7). It should be noted that dietary exposures in the 2003/04 NZTDS were based on average energy diets for each of the age-sex groups (Vannoort and Thompson, 2005). Some consumers have the potential to have significantly higher exposures, particularly high exposure groups (Section III-4.3).
Male |
Female |
Young Male |
Boy |
Girl |
Child |
Toddler |
Infant |
|
|---|---|---|---|---|---|---|---|---|
Total Hg |
0.74 |
0.60 |
0.74 |
0.74 |
0.46 |
1.1 |
1.3 |
1.3 |
% PTWI |
15% |
12% |
15% |
15% |
9% |
21% |
25% |
26% |
%PTWI (MeHg) |
46% |
38% |
46% |
46% |
29% |
69% |
81% |
81% |
Vannoort and Thompson (2005) estimate that the weekly dietary intake exposures for all age-sex groups was 26% or less than the PTWI for total mercury, and 86% or less if all of the mercury is assumed to be methylmercury. The NZFSA also states that the PTWI has safety factors built into it and the PTWI represent a level of no appreciable risk for a lifetime exposure. Overall, the NZFSA assessment is that the New Zealand population is highly unlikely to have any adverse health effects as a result of dietary exposure to mercury.
As noted earlier, there are likely to be regional differences in the concentration of methylmercury in some freshwater fish species. Fish caught anywhere on the North Island’s volcanic plateau (e.g. the Taupo area and in parts of the Waikato River) tend to have methylmercury levels up to ten times higher than their marine counterparts (Section III-3.2.2). This is a result of naturally high concentrations from discharges of geothermal waters. The NZFSA is currently investigating whether fish from certain marine fisheries may contain higher concentrations of methylmercury from geothermal discharges, e.g. Bay of Plenty due to geothermal inputs from White Island.
NZFSA notes that some high fish-eating consumers have the potential to have significantly higher exposures and that it may be advisable for toddlers, infants and women of child bearing age to limit the types of fish they eat and the frequency that it is eaten. However, while some fish are known to contain elevated concentrations of mercury, fish is also an important source of nutrition (Diez, 2008). Fish contain nutrients important for brain development, such as omega-3 fatty acids (WHO, 2008) as well as iodine and vitamins. Based on both the strength of the evidence and the potential magnitudes of effect, the benefits of fish intake exceed the potential risks from mercury (Tsuchiya et al, 2008). For women of childbearing age, benefits of modest fish intake, excepting a few selected species (Table III-9), outweigh risks from mercury (Mozaffarian and Rimm, 2006). NZFSA (2009) recommends eating fish during pregnancy as part of a well-balanced diet.
4.2.2 Drinking Water
Generally, mercury concentrations in drinking water supplies are below the analytical detection limit. Davies et al, (2001), in reporting on the chemical quality of community water supplies in New Zealand, detected total mercury (at a detection limit of 0.0005 mg/L) in only three drinking water zones (3% of those assessed) with a detection at greater than 50% of the then Maximum Acceptable Value (MAV) of 0.002 mg/L (now 0.007 mg/L) in only one zone, the latter representing an estimated 100 people.
As a conservative estimate of drinking water intake, Cavanagh (in prep) proposed using 10% of 0.002 mg/L, i.e. 0.0002 mg/L. For the normal assumption of an adult drinking two litres of water per day, this results in an intake of 0.4 µg/day, or 0.04 µg/kg bw/week (less than 1% of the PTWI). This is very conservative when compared to the actual concentration of mercury in the Waikato River at Lake Ohakuri, downstream of several major geothermal discharges, of less than 0.00008 mg/L (giving rise to an intake of less than 0.16 µg/day for a water consumption of two litres). The Waikato River is the source of several community water supplies, including Hamilton and a proportion of Auckland’s. However the concentrations of mercury in municipal drinking water supplies are generally lower than in their source waters because a substantial part of any mercury that is present is removed during the standard water treatment process.
Overall, drinking water is thought to be an insignificant exposure route for mercury exposure for most people in New Zealand.
4.2.3 Dental Amalgams
The amount of mercury absorbed into the body from amalgams is likely to be idiosyncratic, being higher in people with more amalgam fillings, particularly if the fillings are of poor quality. As noted in section II, WHO (1991) estimated that for an average person with 8-10 amalgam fillings, between 21 and 119 µg of mercury might be absorbed per week as a result of loss of mercury from the amalgams. For a 70 kg adult, this translates to 0.3 to 1.7 µg/kg bw/week, or 6 to 34% of the PTWI.
4.2.4 Inhalation of Air
A typical concentration of mercury in air in New Zealand has been estimated at between 0.42 ng/m3 and 3.1 ng/m3 (Bibby and Patterson, 1988, de Mora, et al, 1991, Fellows and Bates, 1998). If it is assumed that the average adult inhales approximately 22 m3 of air per day (Fellows and Bates, 1998), an average adult will have an inhalation exposure to mercury of 0.07 µg/day or 0.48 µg/week. This translates to 0.007 µg/kg bw/week, or 0.1% of the WHO PTWI for total mercury.
Around Rotorua, mercury emission rates have been measured at up to 20 ng/m3 (Fellows and Bates, 1998). Even at these concentrations, the total intake of mercury is only 3.08 µg /week or 0.044 µg/kg bw/week for a 70 kg adult, which is less than 1% of the PTWI. Therefore inhalation of mercury in air is unlikely to be a significant exposure route.
4.2.5 Smoking
A significant sub-group of the population is exposed to mercury through smoking. Fowler et al (2000) has estimated that smoking an average cigarette exposes a smoker to 5 mg of mercury per cigarette. Assuming a smoker smokes 10 to 20 cigarettes per day, the smoker could be exposed to between 0.35 to 0.7 µg of mercury per week. For a 70 kg adult this translates 0.005 to 0.012 µg/kg bw/week, a small fraction of the PTWI.
4.2.6 Summary of Estimated Exposure for General Population
Based upon the exposure scenarios above, it is estimated that the ‘average’ 70 kg adult New Zealander is exposed to between 76 and 174 µg Hg/week (1.1 – 2.5 µg/kg bw/week), or approximately 22 to 50% of the PTWI for total mercury (Table III-8). Assuming that the methylmercury concentration is approximately 80% of the total mercury measured in fish (Diez, 2008), the ‘average’ New Zealander is exposed to roughly 42 µg/week of methylmercury, or approximately 42% of the PTWI for methylmercury.
The consumption of fish is generally the most important exposure pathway in terms of human exposure to total mercury and methylmercury. However, for people with a large number of mercury amalgam fillings, exposure to mercury from these fillings may also be a significant source.
These calculations will underestimate a person’s exposure if they consume a large amount of seafood in their diet, particularly if they consume significant quantities of fish species which are high in mercury (Table III-9), or if they have a significant occupational exposure to mercury (Section III-4.3).
| Source of Exposure | Elemental Mercury | Inorganic Mercury | Methylmercury | Total Mercury Exposure |
|---|---|---|---|---|
| Air | 0.48 µg/week | 0.48 µg/week | ||
| Food - Fish |
10 µg/week | 42 µg/week | 52 µg/week | |
| Drinking water | 2.8 µg/week | 2.8 µg/week | ||
| Dental Amalgams | 21-119 µg/week | 21-119 µg/week | ||
| Total | 31-130 µg/week | 2.8 µg/week | 42 µg/week | 76-174 µg/week |
| PTWI1 | 112 µg/week | 350 µg/week | ||
| % PTWI | 37.5% | 22%-50% |
1 Exposure assumed for 70 kg adult
4.2.7 Broken Compact Fluorescent Lamps
Exposure to mercury from broken compact fluorescent lamps (CFLs) results in a short term exposure. Studies undertaken by the New Jersey Department of Environmental Protection (Aucott et al, 2003) found about one third of the mercury in a broken lamp was released within the first eight hours. Assuming that a single CFL has approximately 5 mg of mercury and that the volume of air in a standard size room is approximately 33 m3, a broken CFL could produce air mercury concentrations in the range of 8 to 20 µg/m3 within eight hours after breakage if the room was not ventilated (Groth, 2008). These concentrations exceed the ATSDR (1999) MRL (0.2 µg/m3) and WHO (2000) ambient air concentration (1.0 µg/m3) guideline levels.
Mathematical modelling of mercury emissions from a broken CFL by the Florida Department of Environmental Protection (Chandrasekhar, 2007) found that by ventilating the room and using a fan to increase the air flow, the airborne mercury concentrations were reduced to below the ATSDR MRL within 12 minutes and were at background levels within 20-25 minutes (Chandrasekhar, 2007). Similarly, experiments by the Maine Department of Environmental Protection (DEP) (Maine DEP, 2008), measuring mercury concentration after breaking a hot CFL (when most of the mercury is likely to be in a gaseous phase), found that mercury concentrations generally dropped to below the US EPA inhalation reference concentration (RfC) of 0.3 µg/m3 within minutes, although in some experiments concentrations remained above this value for more than an hour. Short term spikes in excess of 100 µg/m3 were recorded for some experiments. Cleaning by vacuuming increased the concentrations, and re-vacuuming days later after an initial clean-up also remobilised the mercury to above the US EPA RfC.
It should be noted that in both the Maine and Florida studies, the selected health criteria are applicable to long-term continuous exposure. ATSDR (1999) emphasises that the chronic MRL is not intended to be used as an estimation of a threshold level, as exceeding the MRL does not necessarily mean that a health threat exists. ATSDR did not derive acute or intermediate MRLs – values that would be more applicable to exposure from a broken lamp – because of a lack of data. ATSDR (1999) notes that the chronic inhalation MRL is, by definition, a level that is considered to be without appreciable health risk over a lifetime of exposure at that level. It is further considered to be a "safe" level for all the exposed human population, when exposure exists for 24 hours a day, 7 days a week for an extended period of years. People may be able to "tolerate" metallic mercury levels above the MRL for intermittent periods.
The Maine DEP (2008) took a different view to the ATSDR MRL in their reporting of the lamp breaking experiments, regarding even short-term excursions over the MRL as a potential risk, particularly for a young child. This is a very conservative, precautionary approach.
Taking the ASDR (1999) view that each exposure case must be considered on its merits, it is possible to use the Maine DEP data to obtain an idea of an individual’s likely exposure to mercury in the event of CFLs being broken regularly.
It is appropriate, in the first instance, to use a conservative exposure scenario to assess the possible risk. Such an exposure is termed the reasonable maximum exposure estimate (RME). If it is assumed that an average house has 30 mercury-containing lamps, and the lamps have an average life of five years, then a lamp has to be replaced on average every two months. If it is further assumed that on every second replacement the lamp is broken (a conservative assumption) then an individual could be exposed to mercury on three occasions (being the actual event and the lingering effects) per year.
The Maine DEP studies showed that concentrations in air dropped to within the ATSDR MRL quickly – generally within minutes to a few hours. However, scenarios that had initial vacuum cleaning and subsequent regular vacuum resulted in extended period of several hours with average concentrations of around 1 µg/m3. If a concentration of 1 µg/m3 is assumed for the first 24 hours, the same average concentration occurs for 24 hours for two subsequent vacuuming cleanings one week apart and drops to half that concentration on a fourth vacuuming, after which concentrations stay within the ATSDR MRL regardless of vacuuming.
If the same adult is exposed for 24 hours within the room (an unlikely scenario), with the adult breathing at a rate of 15.2 m3/day (US EPA, 1997b), then the average monthly rate of exposure is 13.3 µg. This is the equivalent of 0.19 µg/kg bw/month or 4% of the WHO PTWI.
The risk to a child would greater, given a lower body weight. A child remaining in the same room on each occasion would be unusual, but assuming a body weight of 15 kg for a toddler and an inhalation rate of 6.8 m3/day (US EPA, 1997b) results in an average exposure of 0.4 µg/kg bw/month, or 8% of the PTWI.
It should be noted that the above estimates are conservative estimates and that an individual actual exposure to mercury as a result of breakages of CFLs is probably much lower, probably a fraction of 1% of the PTWI. This maximum likely extreme exposure assessment has been adopted to demonstrate that breakages of CFLs is unlikely to result in an unacceptable exposure to mercury. This exposure assessment supports the finding of a report prepared for the Ministry of Health (TERA, 2008), which also found that likely mercury exposure were also below risk-based acute exposure screening levels.
These studies confirm that there is only a very small risk from breakages of CFLs, a conclusion supported by Groth (2008). However, this risk can be reduce or eliminated if prompt and proper clean up is carried out. The Ministry for the Environment has recommendations on how to clean up mercury released from a broken CFL on its website11.
When dealing a broken CFL it is advisable that children are removed from the room that the breakage has occurred in. This is because children are more sensitive than adults to mercury spills as:
- Mercury vapours are heavy and tend to settle, making mercury concentrations higher at floor level;
- The blood-brain barrier of children is not as effective at keeping mercury out of the brain;
- The respiration rate of children is higher than that of adults, so children inhale more mercury at a given concentration than adults do; and
- The nervous system of children is still developing and therefore more sensitive to the effects of mercury toxicity (Baughman, 2006.)
4.3 Populations with Potentially High Exposures
Some people are more susceptible to mercury toxicity. The WHO identifies two main susceptible sub-populations (WHO, 2008). These are:
- people who are more sensitive to the effects of mercury exposure; and
- those who potentially are exposed to higher concentrations or have a higher dietary intake of mercury (WHO, 2008).
4.3.1 Potentially Sensitive Populations
Foetuses, new born babies and infants are particularly sensitive to the effects of mercury as their blood-brain barrier is less developed than adults and is less effective at preventing mercury from reaching the brain. Excessive exposure can result in neurological effects (WHO, 2008). Besides being potentially exposed to mercury in the uterus, new born babies can be further exposed to mercury from drinking contaminated breast milk (WHO, 2008). It should be noted that the NZFSA does not consider breast milk as a significant source of mercury in New Zealand mothers and the benefits of breastfeeding far outweigh any risks posed by the small amount of mercury that may be present in breast milk (NZFSA, 2009).
Due to the risk that methylmercury poses to the developing foetus and infants, the WHO (2008) recommends that new mothers, pregnant women, and women who might become pregnant should be aware of the hazards associated with methylmercury. To minimise the potential effects on unborn children the NZFSA recommends that pregnant women should reduce their intake of some longer lived and larger fish (NZFSA, 2009). The NZFSA also recommends that consumption of fish from geothermal areas should be limited to no more than one serving per week or fortnight (NZFSA, 2009). A summary of the recommended intake of various fish species in New Zealand is outlined in Table III-9.
| No restriction necessary | 3 – 4 servings12 per week acceptable | 1 serving per 1 – 2 weeks acceptable |
|---|---|---|
| Anchovy Arrow squid Barracouta Blue cod Brill/Turbot Brown trout from Lake Ellesmere Cockles Eel, long or short finned Elephant fish Flounders Gurnard Hoki John Dory Monkfish or stargazer Mussels (green and blue) Orange perch Oysters (Bluff* and Pacific) Parore Rainbow trout from non-geothermal regions Salmon (farmed) Scallops Skipjack tuna Sole (except Lemon sole) Southern blue whiting Surf clams (e.g. tuatua) Tarakihi Toothfish (Antarctic) Warehou (common, silver and white) Whitebait (Inanga) |
Albacore tuna Alfonsino Bass Bluenose Gemfish Ghost sharks Hake Hapuka (Groper) Javelin Fish Kahawai Kingfish Lake Taupo trout Leatherjacket Lemon sole Ling Mackerel (blue and jack) Orange roughy Oreo dories Red cod Ribaldo Rig (Lemonfish, Spotted dogfish) Rock lobster Sea perch Silverside Skate Smooth oreo Snapper Sprats Trevally |
Cardinal fish Dogfish (excluding rig) Lake Rotomahana trout Lake trout from geothermal regions School shark (Greyboy, Tope) Marlin (striped) Southern bluefin tuna Swordfish |
Source: NZFSA (2009).
4.3.2 Populations with Greater Exposure to Mercury
In addition to people who are occupationally exposed to mercury (such as dental professionals), there are several groups within the general population with potentially high exposures (i.e. higher than background) to mercury (ASTDR, 1999). As eating fish and seafood is the major exposure pathway of mercury, individuals who eat a large amount of seafood have a greater exposure to mercury (especially methylmercury). Individuals who may have a higher exposure to mercury include recreational anglers and subsistence fishers. Concentration of methylmercury in fish caught from geothermal lakes and the Waikato River hydro lakes are elevated (Section III-3.2.2). Methylmercury concentrations in sport fish (i.e. swordfish, marlin, sharks, etc) can be at least an order of magnitude higher than in commercial fish purchased from the supermarket (ATSDR, 1999).
People living near waste disposal sites (landfills) which operate landfill gas flares, and waste transfer stations, may be exposed to mercury through several exposure pathways, including inhalation, dermal contact and oral exposure (ATSDR, 1999). Currently there is insufficient information to determine the potential exposure risk for these populations.
People who use cosmetic products and medicinal products containing mercury are exposed to higher mercury levels than the general population (ATSDR, 1999). Mercury is not used in pharmaceuticals and cosmetic products sold in New Zealand, although some ethnic medicines (such as some Chinese traditional medicine, e.g. Fufang Luhui Jiaonang,which contains between 11-13% mercury)) and cosmetics (i.e. skin lighting creams and soaps) are imported into New Zealand and used by some individuals. Mercury-containing skin lightening soaps and creams are left on the skin overnight and the mercury contained within them can be absorbed through the skin (WHO, 1991). The evidence indicates that the total exposure to mercury is substantial from these sources (WHO, 1991).
4.4 Occupational Exposures
Worldwide, most occupational exposure to mercury involves exposure to inorganic or metallic mercury forms such as mercury from chemical processes (e.g. chlor-alkai production), electrical equipment, thermometers, metal processing and in medical and dental services (Hu, 2000). Mercury is still used in New Zealand in a number of occupational settings (such as dentistry, medical applications, commercial/research and school laboratories) and aryl mercury compounds are used in printing inks and resin manufacturing (Luckman and Slaney, 2005). There is no longer any chlor-alkali production in New Zealand.
In New Zealand between March 1992 and June 1998 there were four confirmed cases of work related mercury poisoning (Driscoll et al, 2004). However, no information on the type of exposure or the industrial setting that exposure occurred in was provided in the report.
It is likely that dentists and dental workers have the highest occupational exposure to mercury in New Zealand. A study conducted by Richardson (2003) found that the current single largest source of mercury exposure to a dentist is the removal of old amalgam fillings, which can increase the inhalation of mercury vapour within the dental surgery over the course of an entire week.
Potential occupational exposure may also exist for sanitation workers and employees involved in recycling fluorescent lamps (Aucott et al, 2003). Southworth et al, (2005) measured mercury emissions at transfer facilities and from waste bins in the field and found surprisingly high rates of mercury emissions (up to 100 mg/hr at some transfer stations). Up to 20-40% of mercury emissions from waste disposal processes may occur during the collection, transfer station activities and waste storage before landfilling (Southworth, et al, 2005). Aucott et al, (2003) suggested that airborne concentrations of mercury may exceed occupational exposure limits at some waste handling facilities under certain conditions.
12 A serving is approximately 150 g of fish.
