Air monitoring, dusts and fumes

Lesson 9 showed why the air in a confined space can kill in seconds. Most of the airborne hazards on a construction site work more slowly. Silica dust from cutting and coring concrete, fumes from soldering and welding, solvent vapor from adhesives and coatings, carbon monoxide from fuel-burning equipment, and in older buildings asbestos fibers and mold spores, cause illnesses that show up months, years or decades later: silicosis, lung cancer, chronic obstructive lung disease, mesothelioma, nervous system damage. Because the harm is delayed, these hazards are easy to ignore on the day, and that is exactly why they need deliberate control.

This lesson covers how airborne hazards are measured, starting with the portable gas monitor that every confined space entry depends on, and then the main dusts and fumes a mechanical apprentice meets and how they are controlled.

The portable gas monitor

The instrument used most often on construction sites is a portable multi-gas monitor, usually a four-gas monitor that measures oxygen, flammable gases as a percentage of the lower explosive limit (LEL), carbon monoxide and hydrogen sulfide. It samples the air either by diffusion, letting air reach the sensors on its own, or with a pump that draws air through a tube, which is how a manhole or pit is tested from outside before entry. The monitor shows each reading and sounds an alarm, with lights and vibration, when a reading crosses a set point.

The lower explosive limit is the lowest concentration of a gas in air that can burn. For methane it is about 5 percent by volume. A reading of 10 percent LEL means the methane is at a tenth of that, about 0.5 percent by volume: not yet flammable, but a sign that something is producing gas and that the space must not be entered until it is ventilated and the source is found.

# Compare a set of readings from a pit with the monitor's alarm set points
alarms = {"O2 low %": 19.5, "O2 high %": 23.5, "LEL %": 10, "CO ppm": 35, "H2S ppm": 10}
readings = {"top": (20.8, 0, 2, 0), "middle": (20.4, 2, 3, 4), "bottom": (18.9, 4, 3, 14)}
for level, (o2, lel, co, h2s) in readings.items():
    problems = []
    if o2 < alarms["O2 low %"]: problems.append("low oxygen")
    if o2 > alarms["O2 high %"]: problems.append("oxygen enriched")
    if lel >= alarms["LEL %"]: problems.append("flammable")
    if co >= alarms["CO ppm"]: problems.append("carbon monoxide")
    if h2s >= alarms["H2S ppm"]: problems.append("hydrogen sulfide")
    print(f"{level:7s} O2 {o2:4.1f}%  LEL {lel}%  CO {co} ppm  H2S {h2s} ppm  ->", ", ".join(problems) or "within set points")

top     O2 20.8%  LEL 0%  CO 2 ppm  H2S 0 ppm  -> within set points
middle  O2 20.4%  LEL 2%  CO 3 ppm  H2S 4 ppm  -> within set points
bottom  O2 18.9%  LEL 4%  CO 3 ppm  H2S 14 ppm  -> low oxygen, hydrogen sulfide

The readings in this example came from the ejector pit in lesson 9's story. At the top the air looked fine. Halfway down, hydrogen sulfide was starting to appear. At the bottom, oxygen was low and hydrogen sulfide was above the alarm. A worker who tested only at the cover would have climbed into a pit where the bottom layer could knock them down.

Bump tests and calibration

A gas monitor is only trustworthy if its sensors respond. Sensors can be damaged by exposure to high gas levels, silicone vapors from sealants, moisture and age, and a dead sensor still shows a reassuring zero. Two checks keep monitors honest.

On Kingsway Tower, the monitors live in a docking station in Stonebridge's trailer that bump tests and calibrates them automatically and keeps a record. A monitor that fails its bump test is locked out of use until it is calibrated and passes. Maya is not yet trained to use a monitor for an entry, but she has been shown how to read one and to recognize the alarm, because she may be working near an entry or in a space where a monitor is clipped to someone's collar.

Ventilation

The main way to make the air in a space safe is ventilation: blowing fresh air in or drawing contaminated air out. For confined spaces, the usual method is a blower outside the space pushing fresh air through a flexible duct to the bottom of the space, which pushes contaminated air out through the opening. The blower's intake must be in clean air, away from vehicle exhaust and generators, or it will pump carbon monoxide into the space. Ventilation must run continuously during the entry, and the monitor stays on, because the work itself can produce gas.

Never ventilate a space with pure oxygen. It does not make the air safer; it creates an oxygen-enriched atmosphere in which clothing and materials catch fire readily.

Silica dust

Crystalline silica is found in concrete, block, brick, mortar, stone and many other construction materials. Cutting, coring, grinding, chipping and drilling these materials releases fine respirable dust that can reach deep into the lungs, where it causes silicosis, an incurable scarring of the lungs, and increases the risk of lung cancer, chronic obstructive pulmonary disease and kidney disease. OSHA's construction silica standard, 1926.1153, sets a permissible exposure limit of 50 micrograms per cubic meter averaged over eight hours. Employers can comply by following Table 1 in the standard, which lists common tasks such as core drilling and handheld sawing with the engineering controls and respirators required for each, or by measuring exposures. The standard also requires a written exposure control plan and a competent person to carry it out.

Dry sweeping and blowing dust off clothes with compressed air are two habits that put more silica in the air than the original task. Kingsway Tower bans both. Maya cleans up with the HEPA vacuum and a damp rag, and leaves her dusty coveralls at the site.

Fumes from soldering, brazing and welding

Heating metals and fluxes produces fumes: fine particles of metal oxides and decomposed flux. Soldering copper water lines with lead-free solder, which is required for drinking water systems in both Canada and the United States, produces relatively small amounts of fume, mostly from the flux. Brazing and welding produce more, and welding on galvanized steel or stainless steel produces zinc oxide, which causes a flu-like illness called metal fume fever, or hexavalent chromium, which can cause cancer.

The controls follow the hierarchy: substitute where possible (press fittings instead of solder), ventilate with a fan or local exhaust that pulls the fume away from the worker's breathing zone, position yourself upwind of the plume and keep your head out of it, and use respiratory protection where ventilation is not enough. Fluxes can also irritate the skin, which is why Maya wears gloves when applying them.

Carbon monoxide from equipment

Carbon monoxide is produced by any fuel-burning equipment: propane heaters used to keep concrete and workers warm in winter, gas-powered saws and pumps, generators and vehicles. It is colorless and odorless, and in an enclosed parking level or basement it builds up quickly. Symptoms start with headache, dizziness and nausea, and can progress to confusion, collapse and death. Many construction workers have been poisoned using a gas-powered saw indoors for just a few minutes.

On Kingsway Tower, gas-powered tools are not allowed inside the building; electric and battery tools are used instead. Propane heaters are used only with ventilation, and Stonebridge places fixed CO monitors on the parking levels in winter. If Maya gets a headache or feels dizzy while working near heaters or engines, she gets to fresh air and tells her supervisor; the symptoms are a warning, not something to push through.

Asbestos and other hidden hazards in older buildings

Kingsway Tower is new, but Northbridge also works in older buildings, and Maya will be sent to renovation jobs during her apprenticeship. Buildings constructed before the 1980s may contain asbestos in pipe insulation, boiler insulation, floor tiles, drywall joint compound, plaster and many other materials. Disturbing asbestos releases fibers that cause asbestosis, lung cancer and mesothelioma, often decades later. OSHA's construction asbestos standard, 1926.1101, classifies asbestos work by the type of material and activity and sets controls, training and medical surveillance; building owners and employers must identify asbestos-containing material before work that might disturb it.

The rule for a new worker is clear: if you find insulation, tile or other material you suspect might contain asbestos, and nobody has told you it has been tested, do not disturb it. Stop, keep others away and tell your supervisor. The same applies to suspected mold, lead paint and pigeon droppings, each of which needs its own precautions.

Know the early signs

Monitors and controls are the main defense, but your own body often gives the first sign that something is wrong. The early symptoms of many airborne exposures are easy to mistake for a hangover, a cold or a long day. Learning them, and acting on them instead of pushing through, has saved many workers.

When Maya felt light-headed on her second day of gluing PVC in a small, closed washroom, she remembered this table, stepped out into the corridor and told Luis. They propped the door open, set up a fan to exhaust to the window and took turns doing the joints. Her headache was gone within twenty minutes.