CIH common mistakes
CIH · Understand the traps and how to avoid them.
Computing a full-shift TWA by simply averaging all sample results together when the samples cover unequal time periods (e.g., averaging a 2-hour, a 3-hour, and a 3-hour sample as three equal numbers).
Candidates memorize 'average the results' from textbook examples where every sample is the same length, then apply it mechanically to a scenario with mixed durations.
What to do instead
Before any arithmetic, build a time-weighted table: sum (Ci x Ti) and divide by the total sampled time. Verify the durations add up to the shift length the question specifies, and state the assumption if the stem leaves a gap.
Converting between ppm and mg/m3 using the wrong molar volume, or dropping the conversion entirely when the exposure limit and the sampling result are in different units.
Both 22.4 L (0 degC) and 24.45 L (25 degC) appear in study material, and under exam time pressure the candidate grabs whichever number surfaces first.
What to do instead
Write the conversion as a dimensional-analysis chain with units cancelling at each step, and default to 24.45 L/mol at 25 degC and 760 mmHg unless the stem states otherwise. Flag the answer if the result differs by roughly 9 percent from another option, which is the signature of a 22.4 vs 24.45 error.
Selecting a respirator or other PPE as the correct answer whenever the scenario describes an overexposure, even though the same scenario says substitution or local exhaust ventilation is feasible.
PPE answers feel 'protective' and immediately actionable, so they attract candidates who have not first run the scenario through the hierarchy of controls.
What to do instead
Before reading the options, rank the feasible controls in the stem from most to least effective. Eliminate any option that is PPE unless the stem explicitly says engineering controls are infeasible, already installed, or the exposure is short-term and interim.
Declaring a control 'effective' because it was installed and is running, without any verification step such as smoke-tube testing, hood static pressure, or post-installation exposure monitoring.
Candidates conflate 'control present' with 'control working,' a shortcut reinforced by questions that only describe the hardware.
What to do instead
Treat any stem without performance data as unverified. Prefer the answer that includes a verification or reassessment step (smoke tubes, capture velocity measurement, repeat personal sampling) over one that assumes the design intent was achieved.
Applying the same model to cancer and noncancer endpoints, for example summing hazard quotients across chemicals that affect different target organs, or treating a hazard quotient as if it carried a probability of harm.
Both calculations start from an inhaled dose and both produce a single number, so the two frameworks blur together under time pressure.
What to do instead
Name the endpoint first. Noncancer: threshold model, HQ = exposure divided by the reference concentration, HQ above 1 is a screening flag not a probability, and HQs are only summed for the same target organ. Cancer: linear low-dose model, risk = exposure times unit risk, and risks are summed across all carcinogens.
Using the wrong averaging time or exposure-duration defaults, such as applying a 24-hour-per-day, 365-day-per-year community assumption to a worker, or defaulting to 250 days per year and 8 hours per day when the stem describes a different schedule.
The formulas look identical regardless of scenario, and candidates reach for the memorized defaults instead of re-deriving them from the stem.
What to do instead
Before computing, write out the four exposure factors explicitly (concentration, hours per day, days per year, years) and confirm each against the scenario rather than a habitual default. If the stem is silent, state the default in the answer rationale.
Comparing a measured ambient air concentration in a neighborhood directly against an occupational exposure limit to conclude whether residents are at risk.
PELs and TLVs are the numbers candidates know best, so they get used as a universal yardstick regardless of who is exposed.
What to do instead
Match the limit to the population: occupational limits for healthy adult workers on 8-hour shifts; ATSDR minimal risk levels, EPA reference concentrations and unit risks, or state air toxics standards for the general public, including sensitive subgroups and longer averaging periods. If the answer choice applies an occupational limit to residents, eliminate it.
Choosing the technically optimal remedy when the stem asks what to do first, or choosing an action that ignores the audience and role named in the stem, such as answering a community risk-communication question with a data-collection plan.
Industrial hygienists are trained to prize technical rigor, so the most thorough answer looks best even when the question is testing sequencing or communication judgment.
What to do instead
Underline the operative qualifier (first, next, most appropriate, initial) and the actor named in the stem. Answer from that actor's position and that audience's needs: for the public, plain-language risk framing and uncertainty; for management, cost, liability, and decision options.
Treating an IDLH value as a routine operating exposure limit, or answering an emergency scenario with an air-sampling step as the first action rather than isolation, evacuation, and notification.
IDLH sits in the same tables as PELs and TLVs and gets grouped with them mentally, and sampling feels like the 'scientific' first move.
What to do instead
Classify IDLH as an escape-only criterion used for respirator selection and emergency planning, never as an acceptable ceiling. In emergency stems, the first action is always to protect people: isolate the area, evacuate or shelter, establish command, and notify, with sampling only after life safety is addressed.
Answering an ethics item with either 'do as instructed' or 'refuse and resign,' skipping the intermediate steps of documenting the concern and escalating it through the organization before any external disclosure.
Candidates read ethics items as a binary loyalty-versus-integrity choice and overlook that the correct response is usually a specific sequence of escalating actions.
What to do instead
Order the options by escalation: protect health and public welfare first, document the concern in writing, raise it to the immediate supervisor, then higher management, then regulators or affected parties if unresolved. Choose 'resign' or 'go public immediately' only when the stem states that internal escalation has already failed or that imminent serious harm exists.