Introduction

An exposure pathway must, by definition, have five components:

1. a source of contamination (such as deteriorating lead-based paint on the walls, doors and windows of a home; used car batteries; open burning of waste);
2. an environmental medium and transport mechanism (such as lead contaminated dust on the floor of a home, lead smoke from open burning, or lead exhaust from leaded gasoline);
3. a point of exposure (such as children's hands, the floor, or children's toys;
4. a route of exposure (such as eating the dust through hand-to-mouth behavior); and
5. an exposed population (such as children in the home environment or pregnant women in polluted environments or workplaces).

When all five components are present, the exposure pathway is termed a complete exposure pathway [WHO 2010].

The distribution of lead in the environment varies from place to place. Each of the following sources of lead is discussed further in this section.

• Lead-contaminated house dust is the major source of exposure for children in the United States, resulting from higher lead content paint that remains in older buildings.
• Lead occurs in drinking water through leaching from lead-containing pipes, faucets, and solder frequently found in the plumbing of older buildings.
• Lead in lead-glazed food containers may contaminate water, food, and beverages without being seen, tasted, or smelled.
• Lead may be found in and around workplaces where lead is used as well as in waste materials from production processes.
• Lead may still be found in
  • Some commercial products,
  • Some imported jewelry and candies,
  • Children's toys,
  • Cosmetics, and
  • Folk, traditional or home remedies.
• Lead concentrations in soil, air, and water can be especially high near the sites of historic or ongoing mining operations or smelters.
• Lead concentrations in inner city soil can be especially high from accumulation that occurred before the elimination of lead from gasoline in the United States (early phase down/phase out efforts starting in the late 1970s with continuation into the 1990s).
• Lead from exposure to cigarette smoke (including second and third hand tobacco smoke exposures [SHS & THS]) may contribute to increased blood lead levels (BLLs) in children.
• While BLLs over time are consistently declining in the United States, it is still a serious health problem for many people, particularly children in urban areas.

Economic Costs of Childhood Lead Poisoning

"Blood lead concentrations have decreased dramatically in U.S. children over the past 4 decades, but too many children still live in housing with deteriorated high content lead-based paint and are at risk for lead exposure with resulting lead-associated cognitive impairment and behavioral problems" [AAP 2016].

Landrigan et al. 2002a estimated that the U.S. economy incurs $43.4 billion annually in the costs of all pediatric environmental disease.

Childhood lead poisoning alone accounts for the vast majority of these costs, which includes

• Medical costs,
• Disability,
• Education, and
• Parental lost work time.

A more recent study described the loss of economic productivity resulting from reduced cognitive potential and from preventable childhood lead exposure to range from $44.8-$60.6 billion in 2008 [Trasande and Liu 2011].

Evidence continues to accrue that commonly encountered blood lead concentrations, even those below 5 μg/dL (50 ppb), impair cognition; there is no identified threshold or safe level of lead in blood [AAP 2016].

Evidence-based guidance is available for managing increased lead exposure in children, and reducing sources of lead in the environment, including lead in housing, soil, water, and consumer products, and has been shown to be cost-beneficial [AAP 2016]. Primary prevention should be the focus of policy on childhood lead toxicity.

Lead-based Paint in Homes and Buildings

Lead-contaminated household dust is considered the major high-dose source of lead for children in the United States today [AAP 2016; Lanphear et al. 2002]. Much of this is due to the degradation of interior paint, especially in housing built before 1978.

In 1977 the Consumer Product Safety Commission (CPSC) limited the lead in most paints to 0.06% (600 ppm by dry weight) [CPSC 1977]. In 2009 CPSC further limited lead content in household paint and similar surface coatings in children's products, and some furniture, for adult and children, to a concentration of lead not greater than 0.009 percent (90 parts per million) [CPSC 2009]. "Many of the steel bridges in the highway system are still coated with paint that contains up to 50% lead by weight" [DOT 2016a].

Most literature refers to "Lead-based paint," or "Lead Paint" to paint manufactured before 1978, which used to contain higher amounts of lead. Household paint manufactured today may still contain very small amounts of lead.

In addition to degradation of interior paint, lead may be tracked into homes in significant quantities from exterior soil contaminated by historical use of lead in

• Paint (exterior sources),
• Gasoline, or
• Industries (old smelters).

A secondary source of lead exposure for workers' families can take place if workers bring home lead-contaminated dust on their

• Skin,
• Hair,
• Clothes,
• Shoes, or
• Tools.
• Lead may be released through past or ongoing home renovation.
• Even though leaded paint may be covered with non-leaded paint, lead dust may still be released into the home environment by
  • Chalking,
  • Chipping,
  • Friction
  • Impact, or
  • Peeling.

Lead-based paint is present in one-third of the nation's dwellings [ACCLPP 2012].

• Between 83% and 86% of all homes built before 1978 in the United States have lead-based paint in them [CDC 1997a]. A high percentage of these homes are rental properties, a major risk factor for elevated BLLs [ACCLPP 2012]. Rental homes are likely to have less frequent improvements and poor maintenance, thus allowing older lead-containing paint to peel and flake.
• The older the house, the more likely it is to:
  • Contain lead-based paint,
  • Have a higher concentration of lead in the paint, and
  • Have paint that is deteriorated.
• Before 1955, a significant amount of white house paint sold and used was 50% lead and 50% linseed oil. In 1955, manufacturers adopted a voluntary house paint lead-content standard of 1%, but house paint with higher levels of lead continued to be manufactured [Rabin 1989 as cited in AAP 1993].
• The amount of lead allowable in paint was lowered by federal law to 1.0 % in 1971 and then to 0.06% in 1977.

The number of existing U.S. housing units built before 1950, when paint had high lead content, decreased from 27.5 million in 1990 to 25.8 million in 2000 [CDC 2003], and 23.2 million in 2006 [HUD 2011]. Despite the gradual decline in the number of houses containing higher lead content paint, however, it still poses a risk to the millions of children living in these homes.

Lead in Drinking Water

Lead occurs in drinking water through leaching from lead-containing pipes, faucets, and solder frequently found in the plumbing of older buildings.

• Homes built before 1986 are more likely to have lead pipes, fixtures, and solder, although newer homes may also be at risk if they are built in older neighborhoods (see below).
• "Lead service lines" (the water service pipes that connect the water main in the street or "public water main" to the household plumbing system can be made of lead) in the water supply systems of older neighborhoods can leach lead. Therefore, even if household pipes are replaced, there is still the possibility of receiving lead from "lead service lines".
• "Approximately 7 percent of the homes connected to community water systems [in the United States] have a lead service line." "There are about 15 to 22 million Americans nationally served by lead lines" [AWWA 2016].
• If "lead service lines" are replaced, but the household plumbing remains as galvanized iron pipes, there can still be ongoing lead exposure in drinking water.
• Corrosion build up on the inside of galvanized pipes can create the potential for lead to accumulate over time. Acidic water can contribute to the leaching of lead from pipes into the drinking water supply.
• Having a drinking water filtration system (filtered tap water/filtration pitchers) certified by an independent testing organization such as the National Sanitation Foundation (NSF) to remove lead can be a safe drinking water option when instructions on how to use, maintain, and/or replace filters are followed.
• Boiling water will not eliminate lead.

Other potential sources of lead contamination include [EPA 2016a; Mushak et al. 1989, as cited in AAP 1993]

• Brass fixtures (brass faucets or fittings),
• Copper pipes with lead solder for drinking water, and
• Older drinking water coolers and coffee urns.

EPA recommends replacing lead service lines, galvanized pipes or pipes that were soldered using lead, brass faucets/fittings, and the use of filtration systems for tap water [EPA 2016a].

Lead in water has not been identified as the major problem with lead in the United States. However, some areas of the country may have a serious problem with lead contamination in the drinking water distribution system.

Foods and Beverages Contaminated with Lead

Even when lead is not intentionally used in a product, it may contaminate items such as food, water, or alcoholic drinks, such as moonshine whiskey [CDC 1992; Warren and Vaughan 1922]. Lead may contaminate food during

• Production,
• Processing,
• Packaging, and
• Storage.

Production

Production sources may include

• Leaf and root vegetable uptake of lead from soil,
• Deposits of atmospheric lead onto leafy vegetables [Levin et al. 2008; Mushak et al. 1989 as cited in AAP 1993], and
• Grinding or cutting equipment used during processing that may be contaminated with lead.

Packaging

Lead in packaging may contaminate food.

• Bright red and yellow paints and print on bread bags and candy food wrappers from other countries, particularly Mexico, have been found to contain lead [FDA 2014; Levin et al. 2008; Mushak et al. 1989 as cited in AAP 1993].
• Although lead was phased out of cans containing food or drinks in the United States in the 1980's, some imported food or drink cans may still contain lead in the solder.

Storage

Food or beverages may be stored in lead-containing vessels that contaminate the product.

• Even "safe" pottery and ceramic-ware (especially imported) can become harmful if the protective glaze wears off and exposes people to the lead-containing pigments.
• Lead-glazed pottery, particularly if it is imported, is often overlooked as a potential source of lead exposure.
• Wine and homemade alcohol distilled and/or stored in leaded containers may become contaminated.
• Wine or other alcoholic drinks stored in leaded-crystal glassware may become contaminated.

Other

Other sources of food contamination include

• Certain "natural" calcium supplements.
• Food grown in lead contaminated soil from historic use of lead-containing pesticides in the United States (such as lead arsenate used as an insecticide or fungicide in orchards).

Commercial Products

While lead is prohibited from many products in the United States, imported or pre-regulation products may still pose a risk. Lead is still used in commercial products [CDC 2013b; CDC 2013d; CDC 2013e; Levin et al. 2008] and may be found in products such as

• Artificial turf,
• Artificial Christmas trees,
• Automotive batteries,
• Bridge paint,
• Computers,
• Curtain weights,
• Fishing tackle,
• Jewelry,
• Pewter,
• Pool cue chalk,
• Some ceramic glazes,
• Toys (especially antique or imported),
• Leaded wick candles,
• Vinyl lunch boxes, and
• Vinyl miniblinds (made before 1997).

"Traditional" or "Folk" Imported Home Remedies and Cosmetics

Some people can be exposed to lead using certain imported home remedies or cosmetics [CDC 2013c]. Several examples are listed below.

The Mexican folk remedies azarcon and greta used to treat the colic-like illness "empacho" contain lead. These remedies are also known as

• Alarcon,
• Coral,
• Liga,
• Maria Luisa, and
• Rueda.

Lead-containing remedies used by some Asian communities include

• Ba-baw-san,
• Bali goli,
• Chuifong,
• Ghasard,
• Kandu, and
• Tokuwan.

Middle Eastern remedies and cosmetics that contain lead include

• Alkohl,
• Cebagin, and
• Saoott.

Saper et al. 2004 discussed heavy metal content of Ayurvedic herbal medicine products.

For more information on these products, see the Centers for Disease Control and Prevention's web site, especially Appendix 1 of the document "Managing Elevated Blood Lead Levels Among Young Children" [CDC 2002] at https://www.cdc.gov/nceh/lead/CaseManagement/case Manage_main.htm or
https://www.cdc.gov/nceh/lead/tips/folkmedicine.htm

Environmental and Industrial Sources

Lead is ubiquitous in the environment because of widespread human use. Environmental background levels vary depending on historic and ongoing uses in the area.

• Abandoned industrial lead sites, such as old mines or lead smelters, may continue to pose a potential public health hazard.
• Industrial sources range in size from large mines and hazardous waste sites (e.g., Superfund sites) to small auto repair facilities.
• Industries such as mining and lead smelting contribute to high levels of lead in the environment around such facilities.
• People living near hazardous waste sites, incinerators, landfills may be exposed to lead and chemicals that contain lead by breathing air, drinking water, eating foods, or swallowing dust or dirt that contains lead.
• Local community members may be exposed to lead from these sources through ingestion (or inhalation) of lead-contaminated dust or soils.
• Higher lead content paint may also contaminate soil, especially in areas immediately adjacent to houses built before 1978, and bridges.
• People may be exposed to lead in soils directly or by eating foods grown in lead-contaminated soils, and old orchards.

The past use of lead in gasoline has contaminated soils, especially along roadways. Tetraethyl lead (TEL) was phased out of gasoline for "on-road" use in the United States between 1973 and 1996.

Smoking and Second and Third-hand Smoke

Active and passive smoking (second-hand smoke) is associated with increased BLLs in U.S. adults [Mannino et al. 2005] and children [CDC 2013a]. Third-hand smoke (surface residue from tobacco cigarette smoke) is a new health concern [Ferrante et al. 2013].

Second hand smoke (SHS) is generally considered passive or involuntary inhalation exposure to cigarette smoke by persons other than the intended "active" smoker. It may also be referred to as "Environmental Tobacco Smoke" (ETS). Sometimes ETS is broken down into "mainstream smoke" which is exhaled by the smoker and "side stream smoke" which comes from the end of a lit tobacco cigarette, cigar, or pipe. SHS may contribute to increased BLLs in U.S. children, independent of lead dust concentrations [CDC 2013a; USDA 2009 as cited in CDC 2013a; NHLBI 2007; PHS 2006]. There is no risk-free level of secondhand smoke exposure; even brief exposure can be harmful to health.

Lead dust concentrations, usually ingested during hand to mouth activity, do not appear to mediate this association, suggesting inhalation as a major pathway of exposure from SHS [Apostolou et al. 2012]

Second-hand smoke is associated with increased blood lead levels in U.S. children aged 4-16 years. A representative sample of 5,592 U.S. children, age 4-16 years, who participated in the Third National Health and Nutrition Examination Survey (1988-1994) showed that the geometric mean blood lead levels were 38% higher in children with high cotinine levels compared with children who had low cotinine levels. The logistic regression models showed that children with high cotinine levels were more likely to have blood lead levels ≥10 μg/dL than were children with low cotinine levels [Mannino 2003].

Third-hand smoke (THS) is the residue from tobacco smoke, which remains on just about every surface exposed to that smoke and may build up over time. This residue poses an additional exposure risk, especially in infants and children, that may impact BLLs. Children and infants have an increased exposure risk to tobacco THS than adults because they typically spend more time indoors and have age-specific behaviors (such as mouthing). This increased exposure risk may increase their risk of potential health hazards from exposure to THS [Ferrante et al. 2013]. However, more studies are needed to support an association between third-hand smoke exposure and increased BLLs in children.

Occupational Exposures

The major exposure pathways for workers are inhalation and ingestion of lead-bearing dust and fumes.

Workers in the lead smelting, refining, and manufacturing industries experience the highest and most prolonged occupational exposures to lead [ATSDR 2010]. Increased risk for occupational lead exposure occurs among workers in

• Battery manufacturing,
• Automobile brake repair,
• Companies that work with lead solder,
• Bridge maintenance and repair,
• Construction, especially renovation/rehabilitation,
• Municipal waste incineration,
• Other manufacturing industries,
• Pottery/ceramics companies and studios,
• Radiator repair,
• Rubber products and plastics industries, and
• Steel welding/cutting operations.

Workers renovating highway overpasses and bridges are frequently exposed to higher lead content paint applied to these structures over many years [OSHA 2005].

"Para-occupational", "Take Home", or " Secondary" Exposure

Para-occupational exposure is defined as exposure which occurs in households who live with an occupationally exposed worker, but who are not themselves occupationally exposed. It is important to note that occupational exposures can result in secondary exposure for workers' families if workers bring home lead-contaminated dust on their

• Skin,
• Hair,
• Clothes,
• Shoes, or
• Tools.

Children may also be exposed to occupational lead sources if parents work in these industries and allow their children to visit them at work, or if lead dust from workers'

• Clothing,
• Hair,
• Body,
• Shoes, or
• Tools

contaminates the interior of vehicles that children also ride in.

Many small businesses and cottage industries are actually located in the home. Some examples include

• Battery recycling,
• Jewelry making, and
• Production of leaded glass and pottery.

Household contamination can result in "bystander" exposure levels similar to those found in industrial settings.

Secondary exposures (also called domestic, take home, second hand, etc.) can be prevented by workers showering and/or changing clothing and shoes before returning home or after finishing their work that involves lead at home.

Historical Use of Leaded Gasoline

The past use of lead in gasoline has contaminated soils, particularly in inner cities, and especially along roadways.

"The deadliness of tetraethyl lead was sadly confirmed in the summer of 1924, when workers engaged in producing the additive fell sick and died at several refineries in New Jersey and Ohio. Banner headlines greeted each new fatality until a total of 15 workers had lost their lives -and their minds" [EPA 1985].

Lead has been blended with gasoline, as an additive primarily to boost octane levels, since the early 1920s. In addition to increasing the octane of gasoline, leaded gasoline also protected exhaust valve seats (in vehicles designed to operate on leaded gasoline) from excessive wear. Both of these objectives are now accomplished without the use of leaded gasoline [EPA 1985].

The use of TEL in gasoline paved the way for the development of the high power, high-compression internal combustion engines that were to win WWII and dominate the U.S. automobile industry until the 1970s.

The first reduction standards for Tetraethyl lead (TEL) came out in the early 1970s. Effective January 1, 1996, the Clean Air Act banned the sale of leaded fuel for use in on-road vehicles. However, EPA allowed fuel containing TEL to continue to be sold for off-road uses, including aircraft, racing cars, farm equipment, and marine engines [EPA 1996]. As of January 2015, only Algeria, Yemen and Iraq continue to use leaded gasoline -in addition to unleaded gasoline [UNEP 2015].

Historical Use of Lead Arsenate

Lead arsenate (PbHAsO4) was first used in the U.S. apple orchards in 1892 as an insecticide spray. It was the most heavily used arsenical throughout the United States and worldwide [Schooley et al. 2008; Peryea 1998].

Lead arsenate was used in the United States as an insect growth regulator, insecticide, herbicide and fungicide [EPA 1986]. This insecticide was very popular among farmers because of its effectiveness, low cost, ease of use, and persistence. Over the next 60 years the frequency and amount of lead arsenate applications increased, leading to pesticide resistance. Growers eventually switched to more viable alternates -at the time- such as DDT.

Widespread use of lead arsenate and frequent applications at increasing rates over time contributed to lead and arsenic to accumulate in topsoil, contaminating thousands of acres across the United States" [Schooley et al. 2008; Peryea 1998].

Lead Sources and Contaminated Media

^*Past use of TEL in the United States

^**Leaded paint for residential use (higher than 0.06% Pb content) was banned in 1977 [CPSC 1977]. In 2008, the lead content in paint was lowered to 0.009% by CPSC [CPSC 2009]. Many of the steel bridges in the highway system are still coated with paint that contains up to 50% lead by weight [DOT 2016a].

Likely Global Sources of Lead

Countries with different regulations, standards and practices than the United States, may have lead exposure from

• Backyard smelters,
• Candles (leaded wick),
• Candy (and packaging/wrappers because of the ink used to print the labels),
• Canned products,
• Cottage industries,
• Jewelry,
• Leaded glassware,
• Stained glass paintings and supplies,
• Moonshine alcohol (may be a risk in some countries),
• Poor quality lead glazes,
• Toys,
• Traditional or folk remedies, etc.

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