Lead is a significant contaminant because of its persistence and the widespread exposure of past generations of children to lead from a variety of sources.

Because of lead’s persistence and binding to soil and dust particles, research suggests that soil around heavily trafficked areas is a significant reservoir of lead.

Because of the rapid neuronal growth and extensive synapse formation that occurs in the nervous systems of very young children, exposure to lead prior to age three is particularly hazardous.

The Port Pirie cohort study of South Australian children in the communities around a lead smelter recently concluded that "the cognitive deficits associated with exposure to environmental lead in early childhood appear to be only partially reversed by a subsequent decline in blood lead level. "22

". . .approximately 5% of the children in this country have blood lead levels that are above or around the intervention level."

Improper disposal of mercury-containing items is a significant means by which we have environmental contamination with mercury.

Methylmercury is one of the most hazardous developmental neurotoxicants known and research indicates that the developing fetal brain is most susceptible to its adverse effects.

Metals

Lead

Lead is a significant contaminant because of its persistence and the widespread exposure of past generations of children to lead from a variety of sources. Lead also provides the best-documented example of a contaminant that causes neurotoxic developmental effects associated with low levels of exposure. The effects of low-level lead exposure manifest as a cascade of health problems. These are characterized by overall dysfunction of the central nervous system including developmental deficits that can be observed as lowered IQ, behavioural problems and poor performance in school.

Source: Adapted from Wallace and Cooper (1986) and USEPA (1986). United States Environmental Protection Agency, "Air Quality Criteria for Lead," Volumes III-IV. Environmental Criteria and Assessment Office (Research Triangle Park, North Carolina, 1986) EPA-600/8-83/028dF.

How Are Children Exposed to Lead?

Children are exposed to lead mainly via ingestion, although they can also inhale fine lead particles or dust. Lead exposure can also be in utero. Lead becomes stored in bone and when maternal calcium stores in bone are liberated for fetal skeletal development, mothers who were exposed to high levels of lead throughout life may give birth to infants with congenital lead poisoning.10

Health Canada suggests that Canadian children are most often exposed to lead from food, air and drinking water.11 Pathways of exposure to lead are, however, multimedia and multi-route as this diagram from the New South Wales EPA’s Web site suggests.

Since the removal of lead from gasoline in North America (as of January 1, 1990, for Canada) children’s exposures and hence blood lead levels have declined significantly. A 1994 intergovernmental committee report cautiously estimated that, although levels have been steadily decreasing since the 1970s when unleaded gasoline was introduced, as many as 66,285 urban children in Canada may have blood lead levels greater than the current level of concern (10 micrograms/dL).12

Despite the removal of such a significant source of lead exposure in the environment (i.e., lead in gasoline), there are still many potential ways by which children can be exposed to lead. Because of lead’s persistence and binding to soil and dust particles, researchers have recently suggested that soil around heavily trafficked areas is a significant reservoir of lead.15

Children are also exposed to lead from industrial and household sources. These include primary and secondary lead smelters and various industrial activities that result in lead emissions. In the home, there can be many sources of lead. The most common source of lead now is from old paint and the paint chips and lead dust that can result in significant exposures to children. Other potential sources are from lead solder in plumbing and canned foods, and cigarette smoke. Where homes have older plumbing there is risk of lead contamination of drinking water and prepared foods. Currently, these household sources are the ones that pose the greatest risk to the average child.

However, there seems to be a continual supply of new and unexpected sources of exposure to lead in a variety of consumer products, including items such as some plastic toys, plastic mini-blinds, imported crayons and candle wicks.16

Some imported items and traditional or folk remedies used by people from South and Central American, Asian and Middle Eastern countries have also been found to contain high amounts of lead.

The greater exploratory behaviour of children of different ages may bring them into more frequent contact with contaminants in their environment. Exposure to lead via dust (whether from outdoors or from household items that release lead particles) may be the most important route in young children, especially those exhibiting pica and frequent hand-to-mouth behaviour.17 Infants and small children are also particularly vulnerable to exposure via direct ingestion of contaminated soil since they frequently put objects (and their hands) in their mouths. This behaviour, called pica (eating of non-food items, including soil) is common in many toddlers and children.18

Neurodevelopmental Effects from Lead Exposure

Because of the rapid neuronal growth and extensive synapse formation that occur in the nervous systems of very young children, exposure to lead prior to age three is particularly hazardous.

Meta-analyses of the relatively enormous body of scientific data on children’s exposure to lead have determined that in children under age four there is about a two point decrease in IQ with an increase in blood lead from 0.5 to 1.0 micromoles/L (10 to 20 micrograms/dL).19 The current blood lead level of concern is 0.5 micromoles/L (10 micrograms/dL), as established by the US Center for Disease Control in 1991. A growing body of researchers suggests, however, that there are a number of troubling behavioural and cognitive effects from exposure to levels below the concern level, including attention deficit hyperactivity disorder (ADHD) and learning disabilities in school age children.20 It is also generally believed that there is no real "threshold" for these neurotoxic effects from lead, as they can be demonstrated down to blood lead levels of 1 micrograms/dL.21

Questions to Ask

Given that there are a number of potential ways that a child may be exposed to lead, questions must cover the fundamental areas as outlined by the CH2OP framework. To determine whether, and in what way, the child has been exposed to lead, and to ensure that such exposure is eliminated, the following questions are relevant to ask:

The Community

• Are there local sources of potential lead exposure, such as a lead smelter operation or battery recycling plant, etc?

Since the elimination of lead in gasoline in this country in 1990, industrial sources of lead represent a reason we still may find some children with higher than average blood lead levels.

The Home

• What is the age of the home?

Older homes (prior to 1950) are more likely to have lead plumbing or copper pipes with lead soldering, which can affect the lead content of tap water. They are also more likely to have lead painted surfaces. If the home is in disrepair and has peeling paint, there is greater likelihood that the child will be exposed to lead in paint chips or lead dust on surfaces.

• Was there any recent renovation work done?

Depending on the age of the home, this could signal possible exposure to lead dust from old paint.

Hobbies of Household Members

• Is lead used in hobby materials?

Lead solder used in stained glass, certain enamels and pottery glazes have high lead content.

Restoration of old painted furniture may involve exposure to lead paint. If a household member works on automotive repair as a hobby, there are a number of potential sources of exposure to lead from such activities.

Occupational Exposure

• What is the occupational history of the parents?

If parents are exposed to hazardous substances such as lead through their work, this may mean lead dust is brought home if proper clean up is not observed. Exposure to lead may occur to someone in the home renovation business or in auto repair.

Personal Habits

• Does the child exhibit pica behaviour?

Children who have a penchant for eating non-food items, especially soil or dirt, have a greater risk of exposure to lead in their environment.

• Does the child use a pacifier, suck his or her thumb or exhibit frequent hand-to-mouth behaviour?

Such behaviour increases the risk that children will ingest any lead dust that they come in contact with. One study demonstrated that the use of a pacifier, in particular, was associated with increased odds for elevated blood lead, indicating that the pacifier provided perhaps a constant source of exposure to small amounts of lead dust.23

• What is the child’s nutritional status, dietary intake of calcium and iron levels?

Low dietary calcium is associated with increased gastrointestinal absorption of lead, and iron-deficient children are more likely to eat soil which can expose them to contaminants, lead among them.24

Given that a number of unusual sources of lead have been documented of late (e.g., imported crayons, candle wicks, toys, accessories, etc.), the physician may have to probe further if none of the above questions highlights an obvious exposure.

Assessing Exposure to Lead

A test of blood lead levels is the only dependable indicator of a child’s current exposure to lead. Acute lead poisoning is associated with children’s blood lead levels above 3.4 micromoles/L (70 micrograms/dL). At this exposure the patient would require hospitalization as there is likely to be damage to the peripheral nervous system, causing severe health effects such as muscle and abdominal pain, mental symptoms, paralysis, coma and even death. Exposure at such high levels is quite infrequent and unusual these days in Canada.25

There is however, a continuum of neurotoxic symptoms in children linked with blood lead levels well below 3.4 micromoles/L. For blood lead levels between 0.48 and 1.93 micromoles/L (10 and 40 micrograms/dL, respectively), the associated neurotoxic effects, while "clinically invisible," are significant nonetheless.

In Ontario, limited data indicate that currently the average blood lead level for children is about 0.14 micromoles/L (3 micrograms/dL).26 Although these average levels are below the current "concern level," the actual distribution of blood lead levels indicates that approximately 5% of the children in this country do have blood lead levels that are above or around the intervention level.

Who should be tested for blood lead levels?

From Wallace and Cooper, 1986. With permission.

We do not have universal screening for blood lead in Canada. Some sources recommend that if the answer to any of the following questions in this simple personal risk assessment is "yes" or "not sure," the physician should consider a screening test for blood lead levels, especially in children younger than three years.27

1. Does the child spend substantial amount of time in a home or facility that was built before 1950?
2. Does the child spend substantial amount of time in a home or facility that was built before 1978 that has undergone renovations or remodeling during the previous six months?
3. Does the child have a sibling or playmate who has been treated or followed up for lead poisoning?
4. Has the child been seen eating paint chips?

Personal Prevention of Exposure to Lead

• Check the house for sources of lead in plumbing pipes and replace older types (prior to 1989) that may contain or be made of lead (e.g., lead pipes, or old lead-soldered copper pipes, old brass pipes, faucets and fittings).
• Only cold water from taps should be consumed and patients should practice daily "flushing" in the morning. That is, they should run tap water vigorously for a few minutes until water runs cold to reduce the drinking water content of lead that accumulates in standing water.
  
• Before embarking on renovations where removal of old paint may be involved, consult appropriate sources on how best to avoid toxic exposures.
• Parents can instill frequent hand-washing behaviour in their children to reduce their exposure.

Suggested Web Sites

From Health Canada’s "It’s Your Health" series, Home Renovations: Removing Lead-Based Paint

The B.C. Ministry of Health’s web page, Lead Paint in the Home

The US National Safety Council’s Environmental Health Center Lead Program, "Home Repairs and Renovations: What You Should Know About Lead-Based Paint"

Further Recommended Resources

Lead in Your Home, 1997, 23 p.

This booklet, prepared by Health Canada and Canada Mortgage and Housing Corporation, provides readers with information about the effects of lead in and around the home, explaining the risks to infants, young children, pregnant women and the developing fetus. Available in PDF format.

Health Canada’s, It’s Your Health: Lead and Human Health

The Learning Disabilities Association of Canada’s brief, "Watch Out for Lead"

Mercury

Mercury is a substance that occurs naturally in the environment in low levels and in the past was emitted from various industrial processes. Although most such industrial uses have been eliminated or drastically reduced in Canada since the 1970s, there is still residual evidence of mercury contamination in fish near the former sources of mercury pollution and significant amounts entering the aquatic environment from all sources. Elemental mercury is the form of mercury found in thermometers, thermostat switches, fluorescent light tubes, disc batteries and dental amalgam used in tooth fillings. Improper disposal of such mercury-containing items is a significant means by which we have environmental contamination with mercury.

Fish ingest methylmercury (MeHg, the organic, bioavailable form of mercury) from water and it readily accumulates in their tissues. Health Canada releases fish consumption advisories when testing and routine monitoring determine that fish contaminant levels are elevated and/or research demonstrates health effects from levels previously believed to be "safe" for consumption. Mercury contamination is responsible for 99% of the advisories issued for fish from inland waters, whereas Great Lakes fish are more often contaminated with a variety of other pollutants (see POPs below).

Methylmercury is one of the most hazardous developmental neurotoxicants known and research indicates that the developing fetal brain is most susceptible to its adverse effects. There have been a number of populations exposed to high levels of MeHg with tragic consequences (e.g., Minimata, Japan, and Grassy Narrows and Whitedog28 Ojibway communities in Northern Ontario). Epidemiological studies of fish-eating populations exposed to lower levels of mercury (e.g., Seychelles Islands, Faroe Islands) have not come to a consensus on the risks to the fetal brain. However, there is some suggestion that even in low amounts, mercury can delay developmental milestones like walking and talking and may cause learning problems.

How Are Children Exposed to Mercury?

Exposure to mercury varies depending on the form it takes. Elemental mercury, which is a liquid at room temperature, is inhaled as a vapour and absorbed by the lungs. In an acute exposure it can have local effects such as bronchitis and pneumonitis. Long-term exposure to mercury vapour affects the central nervous system primarily, producing early non-specific symptoms such as mild tremor, excitability, insomnia, forgetfulness and anorexia (AAP, 1999). Symptoms progress to more pronounced tremor and a condition known as erethism, marked by diverse behaviour and personality changes.

In 1993, a dozen Hamilton school children between the ages of 9 and 14 found liquid mercury in the laboratory facilities of an abandoned industrial plant.29 Over 250 other children were eventually directly exposed to the mercury during play. Most often, such exposures to elemental mercury are due to accidents.

There has been much controversy regarding the possibility of health effects from exposure to elemental mercury in dental amalgam. The evidence for effects from amalgam fillings suggests that although there is some level of mercury exposure through this route, it is insufficient to cause clinical toxicity except in isolated cases of allergic reactions. Health Canada’s Position Statement on Dental Amalgam and letter "Dear Doctor" outline some key recommendations regarding the use or replacement of dental amalgam fillings in children and pregnant women.

Because it is lipid soluble, organic mercury (e.g., methylmercury) is ingested and absorbed by the gastrointestinal tract. Therefore, mercury contaminated foods, especially fish, are primary sources of organic mercury. Methylmercury can cross the placenta and is released in breast milk, so children can be exposed prenatally and also during infancy when they are breastfed.

In both inhalation and ingestion routes of exposure, there is a high degree (over 80 to 90%) of absorption of mercury by the body.30

Questions to Ask

• Has the child come in contact with possible sources of mercury?

Depending upon the degree of symptoms, an accidental exposure may be suspected. Most mercury-containing devices and equipment have been replaced and this is a rarer avenue of exposure. The amount of mercury in sources, such as an old-style thermometer, is typically too small to produce a clinically significant exposure.31 The physician may also want to ascertain if an adverse reaction to a mercury amalgam tooth filling is likely.

• Does/did the patient consume sport fish or wild game? Did the mother consume sport fish before or during her pregnancy?

Exposure to methylmercury is most commonly from food sources, especially fish that are caught in local waters.

Assessing Exposure to Mercury

• Because methylmercury collects in red blood cells, whole blood concentrations are a possible clue to excess mercury exposure. In the general population blood mercury levels are typically about 1.5 micrograms/dL or less. Five micrograms/dL blood mercury represents an apparent threshold for symptoms of toxicity.
• Mercury levels in hair are also a biomarker of exposure with a level of 1 ppm being a measure typical for the unexposed.

Preventing Exposure to Mercury

Physicians can counsel patients on a number of different choices in their diet and regarding personal activities, particularly if they are considering having children or are pregnant.

• Women of childbearing age and children under age 15, should avoid or limit meals of sport fish and wild game as these may be potential sources of exposure to bioaccumulating, biomagnifying contaminants such as POPs and heavy metals.
• Clearly, patients should not be advised to avoid fish consumption altogether as eating fish provides an important nutritional benefit and fish oils are an unique source of certain essential fatty acids that are important in brain functioning and otherwise beneficial to health. Commercially available, store-bought fish are indeed considered safe sources since they are monitored by government agencies so patients should be reassured that not all fish are considered harmful.
  
• If they routinely eat fish caught for sport, they should be aware of the species and waterways that have the highest contaminant levels. Generally larger species, which are higher up on the aquatic food chain, and older fish, will have the highest levels stored in their fatty tissues. For example, certain species such as walleye and northern pike, which feed on other fish, have higher contaminant levels than species such as whitefish, which feed on non-fish organisms.
• Also, there are recommended cleaning and cooking methods that will reduce the amount of contaminants ingested. In particular, skin and fat of fish should be avoided as prime depots of stored contaminants.
• In Ontario, the Guide to Eating Ontario Sport Fish is published annually in print (distributed free in beer stores and at fishing licence agencies) and online. The guide gives detailed recommendations for reducing exposure to contaminants in sport fish.
• Other provinces have similar resources. For example, in Quebec, Le Fleuve, the online newsletter for the St. Lawrence Vision 2000 Web site, presents reports on risks from eating St. Lawrence marine life. See. for example, "Monitoring poisonings linked to the consumption of shellfish from the St. Lawrence."
• People who eat sport fish must also make themselves aware of special advisories. For example, in order to synchronize with more rigorous Health Canada guidelines for consumption of mercury, the Ontario Ministry of the Environment issued an advisory in July 1998 that women of childbearing age and children under 15 should eat only the least contaminated fish (as outlined in the Guide).
• These two groups should also consume no more than 1 or 2 meals per month of certain types of fish - shark, swordfish and fresh (but not canned) tuna - as these are known to be generally higher in mercury contamination.
• Primary prevention requires that we all be aware of sources of mercury and reduce input to the environment. Therefore as individuals we can dispose of mercury-containing items (such as disc batteries and fluorescent tubes, etc.) at appropriate hazardous waste facilities.

Further Recommended Resources

• The brief entitled, "Mercury in Fish: A Special Advisory for Women of Childbearing Age and Children under 15," is available in PDF format.
• US EPA Factsheet - Mercury Update: Impact on Fish Advisories

PREVIOUS | TOP | NEXT