Lead: sources and health effects

Lead is a soft, dense, gray metal that is easy to melt, shape and alloy. These properties made it useful for thousands of years, and its toxicity has been recognized for almost as long. Today most new uses of lead in consumer products have been phased out, but lead remains common at work: in old paint, solder, batteries, ammunition, scrap and many industrial processes. This lesson explains where workers meet lead, how it gets into the body, where it goes once it is there and what it does. Lesson 8 covers the law and Lessons 9 and 10 cover assessment and control.

Where workers meet lead

Exposure happens in two broad situations: industries that make, use or recycle lead products, and work that disturbs lead already in place, especially old paint.

Lead-based paint deserves special attention. Paint with high lead content was used widely on homes in Canada and the United States until the 1950s and 1960s, with use declining through the 1970s. Lead paint continued to be used much longer on bridges, ships, tanks, industrial equipment and other steel structures because of its corrosion protection. Any building or structure painted before about 1980 should be assumed to contain lead paint until testing shows otherwise. Lesson 9 explains how paint is tested.

How lead enters the body

Workers absorb lead in two main ways.

Inhalation. Breathing lead fume or fine dust is the main route in most workplaces. Fume, produced when lead is heated above about 500 °C by torch cutting, welding or burning, consists of very fine particles that reach deep into the lungs and are absorbed efficiently. Dust from sanding, scraping and blasting is coarser, but much of it is still inhaled and either absorbed in the lungs or cleared up the airways and swallowed.

Ingestion. Swallowing lead dust that gets onto hands, food, drinks, cigarettes, gum or phones is a major and often overlooked route. Adults absorb roughly 10 to 15 percent of ingested lead, more on an empty stomach or with poor nutrition; young children absorb much more, often around 40 to 50 percent. Eating, drinking or smoking in the work area, or before washing, can contribute as much lead as breathing it.

Skin absorption of inorganic lead is very low, though organic lead compounds such as those once used in leaded gasoline are absorbed through the skin. Lead on the skin matters mainly because it is transferred to the mouth and carried home.

Where lead goes

Absorbed lead enters the blood, where most of it binds to red blood cells. From the blood it moves to soft tissues, including the brain, kidneys, liver and nerves, and to bone, where it replaces calcium. In adults, more than 90 percent of the body's lead is stored in bone.

Lead leaves the blood with a half-life of about a month, so blood lead reflects mainly the last few weeks of exposure. Lead in bone has a half-life of years to decades. Bone lead is released back into the blood during pregnancy and breastfeeding, during illness or immobilization and with aging and osteoporosis. A worker who had high exposure years ago may therefore have elevated blood lead long after exposure ends, and a pregnant worker can pass stored lead to the fetus.

Modeling blood lead

A simple one-compartment model shows how blood lead rises during exposure and falls after it stops. Real lead kinetics are more complex because of the bone store, but the model captures the key point: blood lead responds within weeks to changes in exposure.

import math

half_life_days = 30
k = math.log(2) / half_life_days        # elimination rate per day
baseline = 1.0                           # ug/dL background
steady_extra = 34.0                      # ug/dL added at steady state by the job (illustrative)

def blood_lead(day, start=0, stop=120):
    if day < start:
        return baseline
    if day <= stop:
        return baseline + steady_extra * (1 - math.exp(-k * (day - start)))
    peak = baseline + steady_extra * (1 - math.exp(-k * (stop - start)))
    return baseline + (peak - baseline) * math.exp(-k * (day - stop))

for d in (0, 15, 30, 60, 90, 120, 150, 180, 240):
    phase = "exposed" if d <= 120 else "removed"
    print(f"day {d:3d} ({phase:7s}): about {blood_lead(d):5.1f} ug/dL")

day   0 (exposed): about   1.0 ug/dL
day  15 (exposed): about  11.0 ug/dL
day  30 (exposed): about  18.0 ug/dL
day  60 (exposed): about  26.5 ug/dL
day  90 (exposed): about  30.8 ug/dL
day 120 (exposed): about  32.9 ug/dL
day 150 (removed): about  16.9 ug/dL
day 180 (removed): about   9.0 ug/dL
day 240 (removed): about   3.0 ug/dL

In this illustration, blood lead approaches its steady level after about three to four months of exposure and halves roughly every month after exposure stops. In practice, workers with long exposure histories fall more slowly because lead keeps moving out of bone.

Units: µg/dL

US laboratories report blood lead in micrograms per deciliter (µg/dL). Some international and Canadian sources use micromoles per liter (µmol/L); 1 µmol/L equals about 20.7 µg/dL, so 0.48 µmol/L is about 10 µg/dL. Always check which unit a report uses before comparing it with a guideline.

Health effects in adults

Lead affects almost every organ system. The effects depend on how much lead is absorbed and for how long. Effects once thought to need high blood lead levels are now known to occur at much lower levels.

Many early symptoms of lead poisoning are vague and easily mistaken for stress, flu or overwork. Workers and doctors may not make the connection unless they know about the exposure. That is why blood lead testing for exposed workers, covered in Lesson 11, is important: it detects absorption before symptoms appear.

Health effects in children and the developing fetus

Children are much more vulnerable than adults. They absorb more of the lead they swallow, their brains are developing rapidly, and they put hands and objects in their mouths. There is no known safe blood lead level in children; even low levels are associated with reduced IQ, attention problems and behavioral difficulties that may be permanent. The developing fetus is similarly vulnerable, which is why lead exposure of pregnant workers, and of workers who may become pregnant, must be kept very low.

Public health agencies use reference values to identify children with blood lead levels higher than most of the population. The CDC's blood lead reference value for children has been 3.5 µg/dL since 2021, and the same figure is used as a reference value for adults by the Council of State and Territorial Epidemiologists. There is no known safe level for children.

Take-home lead

Lead dust on clothes, boots, skin, hair, tools and in vehicles can be carried home, where it settles in cars, laundry areas, carpets and furniture. Children of lead workers have been found with elevated blood lead from this route. Lesson 10 explains how change rooms, showers, work clothing and vehicle practices prevent take-home exposure. This is one of the most important reasons lead hygiene rules exist.

Recognizing lead poisoning early

Because early lead poisoning looks like many common complaints, supervisors and workers should know the pattern. Signs that should prompt a blood lead test in an exposed worker include unusual tiredness, headaches, irritability or difficulty concentrating; stomach pain, constipation or loss of appetite; aching joints or muscles; and, at higher levels, weakness in the wrists or ankles. A metallic taste or a blue-gray line on the gums is sometimes described but is unreliable. No symptom is specific enough to diagnose lead poisoning: the blood test is the only way to know. A worker who reports any of these symptoms after lead work should be referred to the program physician promptly, and the controls on their tasks should be checked at the same time.