Dose, Route, and the Exposure You Cannot See
Why this matters
The exposures that end a tradesperson's career rarely announce themselves on the day. Silica dust, lead fume, solvent vapor and noise all do their damage below the level anyone notices, and the bill arrives ten or twenty years later as a lung that will not fill, a nervous system that slowed, or a hearing test nobody wants to read. The only way a shop sees that coming is by learning to name the agent, the route it takes into the body, and the number it is being measured against.
Dose is concentration times time, and the body keeps the total
Dose is how much of an agent actually reaches the body: how strong the exposure was, multiplied by how long it lasted, adjusted for how much of it got in. Five minutes in a heavy cloud and four hours in a light haze can deliver the same dose, which is why every serious limit is written as an average over time rather than as a single reading.
Three consequences follow, and all three show up in the field:
- The day you feel fine is not evidence. Most chronic agents give no warning signal at the doses that matter. Silica has no smell. Lead has no taste in the air. Noise that injures hearing at 90 dB is merely annoying.
- Some agents accumulate. Lead is stored in bone and released for years, which is why a blood test reads the running total rather than today's job. Silica particles that reach the deep lung stay there. Others, like most solvents, are cleared within hours, and for those the peak and the daily average matter more than the lifetime.
- Some agents sensitize. A sensitizer is an agent that can make the immune system react to it, after which a tiny exposure triggers a reaction the original dose never did. Isocyanates from spray foam and some wood dusts are the trade examples. For a sensitized worker the useful limit is close to zero, and no time-weighted average protects them.
The four routes in, and the task that opens each
A route of entry is the path an agent takes into the body. The control has to match the route: a glove does nothing for a dust you breathe, and a respirator does nothing for a solvent soaking through your sleeve.
| Route | How it gets in | Trade tasks that open it |
|---|---|---|
| Inhalation | Breathed in as dust, fume, mist, vapor or gas. The fastest and most common route | Cutting, grinding or coring masonry; soldering and brazing; spraying; working in a closed room with solvent or adhesive |
| Skin (dermal) | Absorbed through intact skin, or through cuts and cracked hands. Some agents also burn or sensitize at the surface | Handling solvents, degreasers, pesticides, wet cement, refrigerant oils, contaminated insulation |
| Ingestion | Swallowed, almost always from hands, food, drinks or cigarettes contaminated at work | Eating lunch in the truck after scraping old paint, chewing a pencil after handling lead solder |
| Injection | Driven through the skin by force or a sharp | A pinhole leak in a hydraulic line or an airless sprayer, a needle in a drain or attic, a puncture from contaminated debris |
Two notes on the table. Respirable describes particles small enough to reach the gas-exchange region of the lung, sampled with a cut point of about 4 micrometers; a dust you can see settling on a bench is mostly larger than that, and the respirable fraction is invisible, which is why "it did not look dusty" says little. And a high-pressure injection injury looks like a tiny puncture and is a surgical emergency: anyone with a fluid injection from a hydraulic tool or sprayer goes to an emergency department immediately, telling the staff what fluid and what pressure, however small the wound looks.
Acute and chronic are different clocks
An acute effect follows a short, high exposure and shows up within minutes to days: the headache from carbon monoxide, the burning eyes from mixed cleaners, the dizziness from a solvent in a crawlspace. A chronic effect follows repeated exposure and shows up after months to decades: silicosis, lead's effects on the kidneys and nerves, noise-induced hearing loss, occupational asthma.
The two clocks need different numbers. A short-term limit or ceiling protects against the acute effect; a full-shift average protects against the chronic one. An agent can be well inside its shift average and still deliver an acute hit in one fifteen-minute task, which is the whole reason the vocabulary below has more than one word.
The gap between exposure and disease is latency. A tech who grinds dry for ten years and feels fine is evidence that latency is long, not that grinding dry is fine.
The limit vocabulary, and who writes each number
| Term | What it means | Who sets it | Binding? |
|---|---|---|---|
| PEL (permissible exposure limit) | The legal limit, usually an 8-hour time-weighted average | OSHA. General industry tables at 29 CFR 1910.1000, construction at 29 CFR 1926.55, plus substance-specific standards (lead, silica, asbestos) | Yes, on employers, for employees. A state plan may adopt a lower one |
| TWA (time-weighted average) | Concentration averaged over a stated period, usually 8 hours | A method, not an agency | Only as the basis of a limit |
| STEL (short-term exposure limit) | A 15-minute time-weighted average not to be exceeded during the shift | OSHA for some substances; NIOSH and ACGIH more widely | Binding where OSHA sets one |
| Ceiling | A concentration not to be exceeded at any time | OSHA, NIOSH, ACGIH | Binding where OSHA sets one |
| Action level | A lower trigger, usually half the PEL, that starts monitoring, medical surveillance or training | OSHA, only in substance-specific standards | Yes, as a trigger for those duties |
| REL (recommended exposure limit) | NIOSH's recommendation, usually a TWA for up to a 10-hour day in a 40-hour week | NIOSH, the federal research agency | No. Advisory |
| TLV (threshold limit value) | A guideline for nearly all workers day after day | ACGIH, a private professional body | No, unless a state or contract adopts it |
| IDLH (immediately dangerous to life or health) | The concentration at which escape could be impaired or death follow | NIOSH | No, but it drives respirator selection |
The most important line in the table is the gap between the first row and the sixth. Most of OSHA's general PELs were adopted in 1971 from consensus values of the late 1960s and have barely moved since, and OSHA itself publishes an annotated table showing where newer numbers are lower. The REL and TLV are frequently stricter, sometimes by a factor of two or more. So a shop that keeps under the PEL is legal, and may still be exposing its people to a level the research agency considers harmful.
State layer. States with OSHA-approved state plans can set their own limits, and some do. California's Cal/OSHA keeps its own table in Title 8, Section 5155, which carries lower values than federal OSHA for a long list of substances. If your shop works in a state-plan state, pull that state's table before you compare anything to the federal one.
Which Part. General industry (29 CFR 1910) and construction (29 CFR 1926) can carry different numbers or triggers for the same agent. Service and maintenance usually sit under 1910; installation, alteration and renovation under 1926.
Reading a limit off a safety data sheet
The fastest place a tech meets these words is Section 8 of a safety data sheet, the manufacturer's document required under the Hazard Communication standard. A typical entry, simplified, reads like this:
SECTION 8 - EXPOSURE CONTROLS / PERSONAL PROTECTION
Component OSHA PEL ACGIH TLV NIOSH REL
Toluene 200 ppm TWA 20 ppm TWA 100 ppm TWA
300 ppm C 150 ppm STEL
Skin contact: absorbed; see Section 11
Eng. controls: local exhaust; general ventilation
Respirator: organic vapor cartridge where
limits may be exceeded
Read it in this order: the strictest number in the row (here the TLV, at a tenth of the PEL), whether a short-term or ceiling value exists, whether a skin notation appears, and then the controls. A skin notation means the agent absorbs through skin in amounts that matter, so an air measurement alone understates the dose. See related: HazCom and SDS: Your Right to Know for how the sheet is organized and kept.
One day on a personal monitor, read against every limit
A tech spends a day replacing a commercial-style water heater in an unvented utility room and wears a personal carbon monoxide monitor that logs averages. The log shows three segments:
- 2 hours at 40 ppm, while a neighboring appliance was backdrafting into the room
- 1 hour at 120 ppm, while a gasoline-powered pump drained the old tank through the doorway
- 5 hours at 5 ppm, outside, on the truck and in the yard
The 8-hour TWA. Multiply each concentration by its hours and divide by 8, per 29 CFR 1910.1000(d)(1): 2 times 40 is 80, 1 times 120 is 120, 5 times 5 is 25. The total is 80 plus 120 plus 25, which is 225 ppm-hours, and 225 divided by 8 is 28.1 ppm.
Against each limit.
- OSHA's general-industry carbon monoxide PEL is 50 ppm as an 8-hour TWA. The day passes at 28.1.
- NIOSH's REL is 35 ppm TWA. The day passes at 28.1, with 6.9 ppm of room.
- ACGIH's TLV is 25 ppm TWA. The day fails at 28.1, by 3.1 ppm.
- NIOSH also sets a 200 ppm ceiling. The 120 ppm hour stays below it.
What the average hid. The one-hour segment at 120 ppm delivered 120 of the 225 ppm-hours, just over half the day's total, in an eighth of the day's time. It is more than twice the PEL concentration and more than three times the REL concentration for a full hour. The TWA is legal; the hour was the exposure that gave the tech his headache, and the control that matters is getting the engine out of the doorway, not watching the average. See related: Carbon Monoxide From Your Own Tools for that half of the problem.
What changes the answer.
- A longer day. OSHA's formula always divides by 8. Had the tech stayed two more hours at 40 ppm, the total rises to 225 plus 80, which is 305, and 305 divided by 8 is 38.1 ppm. Still under the federal PEL, now over the REL concentration, and the REL is written for a 10-hour day, so the comparison needs that stated rather than assumed.
- A state plan. Under Cal/OSHA's table the carbon monoxide limit sits below the federal 50, so the same 28.1 could be an exceedance in California while it passes in a federal-OSHA state. Check the current Section 5155 value rather than carrying a number from memory.
- A mixture. Where two agents act on the same organ, OSHA's additive formula at 1910.1000(d)(2) sums each concentration divided by its own limit, and the sum may not exceed 1. Two solvents each at 60 percent of their limits pass individually and fail together at 1.2.
Where the vocabulary misleads
- An air limit says nothing about the other routes. A solvent with a skin notation, or lead on hands at lunch, can deliver a dose that no air sample records.
- A one-time reading is not an exposure assessment. Concentrations swing with the room, the weather and the tool. An assessment covers the task as it is actually done, often more than once. See related: The Exposure Assessment SOP.
- Odor is not a measurement. Hydrogen sulfide deadens the sense of smell at injurious levels.
References
- 29 CFR 1910.1000, Air Contaminants, Tables Z-1 to Z-3 and the TWA and mixture formulas at (d); 29 CFR 1926.55 for construction.
- NIOSH Pocket Guide to Chemical Hazards, for RELs, IDLH values and the carbon monoxide ceiling.
- ACGIH TLVs and BEIs, current edition, and OSHA's annotated PEL tables comparing federal, Cal/OSHA, NIOSH and ACGIH values.
- See related: HazCom + SDS (Chemical Right-to-Know); Protecting Your Hearing, Eyes, and Lungs for the Long Term.
- See related: The Hierarchy of Controls on a Service Call; Respirable Crystalline Silica and the Construction Standard.