Assessing lead exposure

Assessing lead exposure means answering several questions at once. Is lead present in the material being worked on, and how much? How much lead is in the air workers breathe? How much has settled on surfaces, where it can be swallowed or carried home? And how much has actually been absorbed by workers? This lesson covers the tools used to answer each question: paint and material testing, air sampling, surface wipe sampling and, briefly, blood lead monitoring, which Lesson 11 covers in detail.

Step 1: is lead present?

Before work starts, find out whether the materials to be disturbed contain lead. For buildings and structures, the main concern is paint and coatings, but solder, flashing, pipes, ceramic tile glazes and some window putties can also contain lead.

Lead content can be expressed in different units, which causes confusion. Laboratory results for paint chips are usually reported as per cent by weight or as mg/kg, which is the same as parts per million (ppm). XRF reports the mass of lead per area of surface, in mg/cm², which depends on how thick the paint layers are.

def pct_to_mgkg(pct):
    return pct * 10_000          # 1 % = 10,000 mg/kg

for pct in (0.009, 0.06, 0.5, 2.0, 12.0):
    print(f"{pct:6.3f} % by weight = {pct_to_mgkg(pct):9,.0f} mg/kg (ppm)")

# Rough link between weight fraction and areal loading for one paint system
# (illustrative only: real paint layers vary in thickness and density).
thickness_um = 250        # total thickness of all layers, um
density_g_cm3 = 2.5       # density of old lead paint, g/cm3
for pct in (0.5, 2.0, 12.0):
    mass_paint_mg_cm2 = thickness_um * 1e-4 * density_g_cm3 * 1000   # mg of paint per cm2
    lead_mg_cm2 = mass_paint_mg_cm2 * pct / 100
    print(f"{pct:5.1f} % lead in {thickness_um} um of paint is about {lead_mg_cm2:.2f} mg/cm2")

 0.009 % by weight =        90 mg/kg (ppm)
 0.060 % by weight =       600 mg/kg (ppm)
 0.500 % by weight =     5,000 mg/kg (ppm)
 2.000 % by weight =    20,000 mg/kg (ppm)
12.000 % by weight =   120,000 mg/kg (ppm)
  0.5 % lead in 250 um of paint is about 0.31 mg/cm2
  2.0 % lead in 250 um of paint is about 1.25 mg/cm2
 12.0 % lead in 250 um of paint is about 7.50 mg/cm2

Canada's Surface Coating Materials Regulations limit new paint to 90 mg/kg, equal to 0.009 per cent. Older paints often contained several per cent lead or more. For worker protection, many provincial guidelines advise treating any detectable lead as requiring precautions when the paint will be disturbed by methods that create dust or fume, because the exposure depends on the method as much as on the concentration.

Step 2: how much lead is in the air?

Air sampling for lead uses a pump and a 37 mm filter cassette, usually with a mixed cellulose ester filter, worn in the breathing zone. Unlike silica sampling, no cyclone is used, because the lead limit covers inhalable particles. Typical flow rates are 1 to 4 litres per minute. The laboratory digests the filter and analyzes it by inductively coupled plasma or atomic absorption methods, such as NIOSH Methods 7300 and 7303 or OSHA Method ID-121. Lead is easy to detect at low levels, so detection limits are rarely a problem for full-shift samples.

The concentration and TWA calculations are the same as for silica, described in Lesson 4: mass divided by air volume, then time-weighted over 8 hours.

def conc(mass_ug, lpm, minutes):
    return mass_ug / (lpm * minutes / 1000)

samples = [
    ("hand scraping, wet", 6.0, 2.0, 420),
    ("heat gun stripping", 41.0, 2.0, 400),
    ("power sanding, no HEPA shroud", 690.0, 2.0, 380),
    ("torch cutting painted steel", 2950.0, 2.0, 300),
]
for task, m, q, t in samples:
    c = conc(m, q, t)
    flag = "above 50" if c > 50 else ("above 30" if c > 30 else "below 30")
    print(f"{task:32s} {c:8.1f} ug/m3  ({flag})")

hand scraping, wet                    7.1 ug/m3  (below 30)
heat gun stripping                   51.2 ug/m3  (above 50)
power sanding, no HEPA shroud       907.9 ug/m3  (above 50)
torch cutting painted steel        4916.7 ug/m3  (above 50)

These illustrative results follow the pattern seen in published studies: wet hand methods produce low exposures, heat-gun stripping moderate ones, while power sanding without dust collection and torch cutting produce exposures tens to hundreds of times the limit. The method of removal matters more than almost anything else, which is why many rules restrict or prohibit the highest-exposure methods.

Initial assessment

Before starting lead work, Canadian guidance expects employers to estimate the exposure expected from each operation, using air sampling from similar past work, objective data or guidance such as Ontario's construction guideline, which links operations to expected exposure levels. Precautions should match the expected exposure from the start; sampling during the job then confirms whether the controls are adequate.

Step 3: how much lead is on surfaces?

Lead dust settles on floors, windowsills, equipment, lunch tables and vehicles. Surface wipe sampling measures how much is there. A moist wipe is used to wipe a measured area, usually marked with a template, in a set pattern. The laboratory reports the mass of lead on the wipe, which is divided by the area to give a loading in µg/ft² or µg/m².

Wipe sampling is used for several purposes: checking that clean-up after lead work is adequate (clearance), checking that lunchrooms, change rooms and vehicles remain clean, and investigating take-home contamination. US federal clearance levels are expressed in µg/ft²; Canadian guidance often uses the same values, sometimes converted to µg/m². One square foot is 0.0929 m², so 1 µg/ft² is about 10.8 µg/m².

SQFT_PER_M2 = 10.764

dlal = {"floor": 5, "window sill": 40, "window trough": 100}   # EPA 2024, ug/ft2

wipes = [
    ("floor, classroom 2", 0.30, 0.09),     # (location, ug on wipe, area m2)
    ("window sill, classroom 2", 2.9, 0.06),
    ("floor, hallway", 0.85, 0.09),
]
for loc, ug, area_m2 in wipes:
    per_m2 = ug / area_m2
    per_ft2 = per_m2 / SQFT_PER_M2
    surface = "window sill" if "sill" in loc else "floor"
    limit = dlal[surface]
    verdict = "passes" if per_ft2 < limit else "fails"
    print(f"{loc:26s} {per_m2:6.1f} ug/m2 = {per_ft2:5.1f} ug/ft2 -> {verdict} ({limit} ug/ft2)")

floor, classroom 2            3.3 ug/m2 =   0.3 ug/ft2 -> passes (5 ug/ft2)
window sill, classroom 2     48.3 ug/m2 =   4.5 ug/ft2 -> passes (40 ug/ft2)
floor, hallway                9.4 ug/m2 =   0.9 ug/ft2 -> passes (5 ug/ft2)

In this illustration the hallway floor fails a 5 µg/ft² clearance value and must be cleaned again and re-tested. Canada has no national dust-lead clearance standard for occupational settings, so the clearance criteria should be written into the contract and control plan before work begins, often by adopting the US values.

Step 4: how much lead have workers absorbed?

Air and surface sampling show how much lead is around workers. Blood lead testing shows how much they have actually absorbed, by both inhalation and ingestion, so it reflects the combined effect of controls, respirators and hygiene. A worker whose air samples are low but whose blood lead is rising may be eating with contaminated hands, using a poorly fitting respirator or being exposed off the job, for example through a hobby such as casting fishing weights or shooting at an indoor range. Lesson 11 explains blood lead programs.

Putting the assessment together

A full lead assessment for a job usually combines all four steps:

Soil and exterior surfaces

Lead does not stay on the building. Decades of exterior paint weathering, past use of leaded gasoline and earlier careless removal work can leave soil around older buildings, bridges and industrial sites contaminated. Exterior work such as scraping siding, blasting a water tank or demolishing a garage can add more. Soil sampling, usually composite samples from the drip line around a building and from play areas, gives a baseline before work and confirms that the work did not add contamination. Ground sheeting under exterior work, careful collection of paint chips and HEPA vacuuming of the sheeting at the end of each day prevent most soil contamination. Because children play in soil, results near homes, schools and daycares matter most.

Quality in sampling

Lead results can only be trusted if samples are collected and analyzed properly. A few habits make a large difference:

• Use a laboratory accredited for lead analysis in the relevant matrix: air filters, wipes, paint or soil.

• Record exactly where each sample was taken, the area wiped or the air volume, and the conditions at the time.

• Include blank samples, wipes or filters handled the same way but not exposed, to detect contamination of the sampling materials.

• Change gloves between wipe samples and never touch the wipe surface with bare hands.

• Keep a chain-of-custody record so that the results can be relied on if they are challenged.

Good records also allow results to be compared over time and between projects, which builds the objective data that make future assessments faster and cheaper.