I85PREP
⌘ K
STS study materials
STS study resources

STS practice questions

One question at a time. Build your STS understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Hazard Recognition, Evaluation, and Control

According to the hierarchy of controls, which approach is the most effective way to manage a workplace hazard?

  1. Training workers in safe work procedures
  2. Providing personal protective equipment
  3. Posting warning signs at the work area
  4. Eliminating the hazard Correct answer.

Explanation

The hierarchy ranks controls by reliability, beginning with elimination of the hazard, then substitution, engineering controls, administrative controls, and finally PPE. Controls that remove or isolate the hazard at its source do not depend on human behavior to be effective.

Question 2Hazard Recognition, Evaluation, and Control

Which occupational exposure limit represents a concentration that should never be exceeded during any part of the workday?

  1. Permissible exposure limit
  2. Ceiling limit Correct answer.
  3. 8-hour time-weighted average
  4. Short-term exposure limit

Explanation

A ceiling limit is a concentration that must not be exceeded at any moment, so it applies even to brief peaks. A TWA averages exposure over a period such as eight hours, a STEL averages over a short period such as 15 minutes, and a PEL is the regulatory limit itself, which may be expressed as a TWA, STEL, or ceiling.

Question 3Hazard Recognition, Evaluation, and Control

An atmosphere is considered oxygen deficient when the oxygen concentration falls below which value?

  1. 18.0 percent
  2. 23.5 percent
  3. 16.0 percent
  4. 19.5 percent Correct answer.

Explanation

Confined space practice treats air containing less than 19.5 percent oxygen by volume as oxygen deficient, and air above 23.5 percent as oxygen enriched. Both conditions are hazardous and trigger entry controls.

Question 4Hazard Recognition, Evaluation, and Control

A combustible gas meter shows a reading of 25 percent LEL in a confined space. What does this reading indicate?

  1. The concentration is one quarter of the way to the lower flammable limit Correct answer.
  2. The atmosphere is at the upper explosive limit
  3. The concentration equals one quarter of the level at which the mixture will ignite
  4. The gas makes up 25 percent of the space by volume

Explanation

Meter readings expressed as a percentage of the LEL scale the measured concentration against the lower flammable limit, so 25 percent LEL means the concentration is one quarter of the LEL. Many entry programs require action, such as increased ventilation or evacuation, at 10 percent LEL.

Question 5Hazard Recognition, Evaluation, and Control

Under the OSHA noise standard, an increase of how many decibels halves the permissible exposure duration?

  1. 10 dBA
  2. 5 dBA Correct answer.
  3. 6 dBA
  4. 3 dBA

Explanation

OSHA uses a 5 dB exchange rate, so the allowable time is halved for each 5 dBA increase above 90 dBA; 8 hours at 90 dBA becomes 4 hours at 95 dBA. ACGIH and NIOSH use a more protective 3 dB exchange rate.

Question 6Hazard Recognition, Evaluation, and Control

The WBGT index used to assess heat stress combines which measurements?

  1. Relative humidity, barometric pressure, and air velocity readings
  2. Dry-bulb, natural wet-bulb, and globe temperature Correct answer.
  3. Dry-bulb temperature and air velocity only
  4. Radiant heat, air velocity, and dry-bulb temperature

Explanation

WBGT is calculated from dry-bulb, natural wet-bulb, and black globe temperatures, with the globe term weighted more heavily outdoors to account for solar load. The wet-bulb term reflects evaporative cooling capacity and the globe term reflects radiant heat.

Question 7Hazard Recognition, Evaluation, and Control

What primarily determines whether a local exhaust hood captures contaminant released near its face?

  1. The static pressure measured far downstream in the duct
  2. The velocity developed at the point of release Correct answer.
  3. The efficiency rating of the filter installed at the fan
  4. The total volume of the room where the hood is installed

Explanation

Hood performance depends on generating adequate capture velocity at the point where the contaminant is released, because capture velocity falls off rapidly with distance from the hood face. Duct velocity and static pressure must be sufficient to transport captured material, but they do not govern capture at the source.

Question 8Hazard Recognition, Evaluation, and Control

Which practice most effectively reduces a worker's external dose from a sealed gamma radiation source?

  1. Wearing lead-lined gloves while handling the source
  2. Relying on a personal dosimeter for protection
  3. Increasing the time spent close to the source
  4. Maximizing the distance from the source Correct answer.

Explanation

External dose is controlled by time, distance, and shielding, and for a point source the dose rate falls with the square of distance, making distance a powerful and easily applied control. Dosimeters only measure dose and provide no protection.

Question 9Hazard Recognition, Evaluation, and Control

In what order should a confined space be tested before entry?

  1. Combustibles, then toxics, then oxygen
  2. Oxygen, then combustibles, then toxics Correct answer.
  3. Toxics, then oxygen, then combustibles
  4. Oxygen, then toxics, then combustibles

Explanation

Standard practice is to test oxygen first, then flammable gases and vapors, then toxic contaminants. Oxygen is checked first because many combustible and toxic sensors need adequate oxygen to respond accurately, and the oxygen reading also drives respirator selection.

Question 10Hazard Recognition, Evaluation, and Control

Under OSHA's respirable crystalline silica standard for general industry, what is the permissible exposure limit as an 8-hour time-weighted average?

  1. 100 micrograms per cubic meter
  2. 10 micrograms per cubic meter
  3. 50 micrograms per cubic meter Correct answer.
  4. 250 micrograms per cubic meter

Explanation

The standard sets a PEL of 50 micrograms per cubic meter of air as an 8-hour TWA, with an action level of 25 micrograms per cubic meter. Exposures at or above the action level trigger monitoring and medical surveillance obligations.

Question 11Hazard Recognition, Evaluation, and Control

Which section of a safety data sheet contains exposure controls and personal protection information?

  1. Section 11
  2. Section 8 Correct answer.
  3. Section 6
  4. Section 4

Explanation

In the 16-section SDS format, Section 8 covers exposure controls and personal protection, including exposure limits and required engineering controls and PPE. Section 4 covers first aid, Section 6 covers accidental release measures, and Section 11 covers toxicological information.

Question 12Hazard Recognition, Evaluation, and Control

In a typical risk assessment matrix, the risk level assigned to a hazard is based on:

  1. The severity of harm and the likelihood of occurrence Correct answer.
  2. The frequency of inspections and the latest audit score
  3. The type of hazard involved and the applicable regulation
  4. The number of workers exposed and the total cost of the planned controls

Explanation

Risk is characterized by combining the severity of the potential harm with the likelihood that the harmful event occurs. Control decisions then follow from where the hazard falls on the matrix.

Question 13Hazard Recognition, Evaluation, and Control

Where should an air sample be collected to estimate a worker's inhalation exposure?

  1. In the worker's breathing zone Correct answer.
  2. At a fixed sampler near the process
  3. At the center of the room at head height
  4. At the exhaust stack outlet

Explanation

Personal samples are collected in the breathing zone, the area near the nose and mouth, so the result reflects what the worker actually inhales. Fixed-area samples describe general room or source conditions and do not represent an individual's exposure.

Question 14Hazard Recognition, Evaluation, and Control

A HEPA filter is rated to remove what fraction of airborne particles 0.3 micrometers in diameter?

  1. 99.97 percent Correct answer.
  2. 99.999 percent
  3. 99.0 percent
  4. 95.0 percent

Explanation

HEPA filters are rated to capture at least 99.97 percent of particles with a diameter of 0.3 micrometers, which is the most penetrating particle size for that media. Efficiency is generally higher for both smaller and larger particles.

Question 15Hazard Recognition, Evaluation, and Control

Using the OSHA method, what effective attenuation results from a hearing protector with an NRR of 25 dBA when exposure is measured on the A-scale?

  1. 25 × 0.7 = 17.5 dBA
  2. (25 − 7) ÷ 2 = 9 dBA Correct answer.
  3. 25 − 7 = 18 dBA
  4. 25 ÷ 2 = 12.5 dBA

Explanation

For A-weighted exposure measurements, the OSHA method subtracts 7 from the NRR and divides the result by 2 to account for real-world fit and spectral differences. The resulting attenuation is subtracted from the measured TWA to estimate protected exposure.

Question 16Hazard Recognition, Evaluation, and Control

Why is a secondary dust explosion often more destructive than the initial one?

  1. The primary event lofts accumulated dust into a cloud Correct answer.
  2. Secondary explosions occur only in outdoor process areas
  3. Dust layers on surfaces are combustible only in the presence of moisture
  4. The initial blast preheats the air supply to the building

Explanation

The first explosion dislodges dust that has settled on surfaces, beams, and equipment, creating a concentrated cloud that is ignited by residual energy or a still-present ignition source. The resulting secondary explosion can be far more powerful and can propagate through the facility.

Question 17Hazard Recognition, Evaluation, and Control

In safety and health practice, a hazard is best described as:

  1. A potential source of harm Correct answer.
  2. The number of workers exposed to a condition
  3. The severity of an injury that has occurred
  4. The probability that harm will occur

Explanation

A hazard is a condition, substance, or activity with the potential to cause harm. Risk, by contrast, combines the likelihood of that harm with its severity.

Question 18Hazard Recognition, Evaluation, and Control

Before field use, a direct-reading gas meter should be:

  1. Zeroed and calibrated with a known standard Correct answer.
  2. Fitted with a new sensor for each use
  3. Warmed to body temperature before use
  4. Charged overnight before each shift

Explanation

Direct-reading instruments must be zeroed in clean air and checked or calibrated with a known concentration of the target gas so readings are accurate and traceable. Bump tests or calibration checks are performed before each use or per the manufacturer's schedule.

Question 19Hazard Recognition, Evaluation, and Control

Metal fume fever is most commonly associated with exposure to fumes of which metal?

  1. Zinc Correct answer.
  2. Lead
  3. Chromium
  4. Manganese

Explanation

Metal fume fever is an acute, flu-like illness caused by inhaling freshly formed zinc oxide fume, classically from welding or cutting galvanized steel. Symptoms such as fever, chills, and muscle aches typically resolve within a day.

Question 20Hazard Recognition, Evaluation, and Control

Which practice most reliably controls hazardous energy while equipment is being serviced?

  1. Applying a personal lock to an energy-isolating device Correct answer.
  2. Posting warning signs at the electrical panel
  3. Wearing insulated gloves while servicing the equipment
  4. Telling machine operators the equipment is down

Explanation

Lockout/tagout physically isolates equipment from all energy sources and holds it in a de-energized state, so the control does not depend on other people's behavior. Signs and verbal notices are administrative measures that can be overlooked.

Question 21Hazard Recognition, Evaluation, and Control

A worker is exposed to 60 ppm of a solvent for 4 hours and 20 ppm for the other 4 hours of an 8-hour shift. What is the 8-hour time-weighted average exposure?

  1. 35 ppm
  2. 40 ppm Correct answer.
  3. 45 ppm
  4. 50 ppm

Explanation

The TWA is the sum of each concentration multiplied by its duration, divided by the total shift length: (60 × 4 + 20 × 4) ÷ 8 = 40 ppm. Time-weighted averaging balances brief high exposures against longer periods at lower concentrations.

Question 22Hazard Recognition, Evaluation, and Control

Which change most reduces the risk of a musculoskeletal disorder in a repetitive assembly task?

  1. Increasing the production rate to shorten the task
  2. Storing parts on the floor beside the workstation
  3. Adjusting work height and using a power tool Correct answer.
  4. Extending the shift length to reduce the number of workdays

Explanation

Musculoskeletal risk falls when awkward postures, force, and repetition are reduced, so matching work height to the worker and using a power tool addresses posture and force together. The other choices increase repetition, exposure duration, or bending.

Question 23Hazard Recognition, Evaluation, and Control

Under OSHA's lead standard, medical removal is triggered when a worker's blood lead level reaches which value?

  1. 20 micrograms per deciliter
  2. 50 micrograms per deciliter Correct answer.
  3. 80 micrograms per deciliter
  4. 30 micrograms per deciliter

Explanation

The lead standard requires medical removal at a blood lead level of 50 micrograms per deciliter averaged over three months, or 60 micrograms per deciliter on a single test. Removal is protective: the worker retains pay and seniority while removed.

Question 24Hazard Recognition, Evaluation, and Control

Which technique systematically breaks a task into steps to identify hazards before work begins?

  1. Job hazard analysis Correct answer.
  2. Incident investigation
  3. Fault tree analysis
  4. Exposure reconstruction

Explanation

A job hazard analysis lists the steps of a task, identifies the hazards associated with each step, and defines controls before the work is performed. It is prospective, unlike an incident investigation, which is reactive.

Question 25Hazard Recognition, Evaluation, and Control

Dilution ventilation is generally an acceptable control strategy when the airborne contaminant:

  1. Is highly toxic and is released intermittently
  2. Has low toxicity and is released at a uniform rate Correct answer.
  3. Is a fume released from a single point source
  4. Contains a carcinogen and requires strict control

Explanation

Dilution ventilation works by mixing contaminated air with large volumes of clean air, so it suits contaminants of low toxicity released steadily and uniformly throughout a space. Highly toxic, intermittent, or localized releases call for local exhaust or more reliable controls.

Question 26Hazard Recognition, Evaluation, and Control

According to the hierarchy of hazard controls, which action is the most effective way to protect workers from a hazard?

  1. Posting warning signs and training workers
  2. Issuing personal protective equipment to workers
  3. Eliminating the hazard from the workplace Correct answer.
  4. Adding engineering controls at the source

Explanation

The hierarchy ranks controls by reliability, and elimination removes the hazard so that no exposure is possible. Engineering controls are next because they act on the hazard or the pathway without depending on worker behavior. Administrative controls and PPE sit at the bottom because they depend on people following procedures correctly.

Question 27Hazard Recognition, Evaluation, and Control

A confined space atmosphere is considered oxygen deficient when the oxygen concentration falls below which level?

  1. 16.0 percent
  2. 23.5 percent
  3. 19.5 percent Correct answer.
  4. 21.0 percent

Explanation

OSHA defines an oxygen-deficient atmosphere as one containing less than 19.5 percent oxygen by volume. Normal air is about 20.9 percent, so a reading below 19.5 percent signals a deficiency requiring respiratory protection and entry controls. Concentrations below 16 percent are far more dangerous but are not the threshold used to define deficiency.

Question 28Hazard Recognition, Evaluation, and Control

Which statement defines the flash point of a liquid?

  1. The lowest temperature yielding an ignitable vapor mixture Correct answer.
  2. The highest temperature at which vapor will not burn
  3. The temperature at which the liquid begins to boil
  4. The temperature at which a solid ignites without a spark

Explanation

The flash point is the lowest liquid temperature at which enough vapor is generated above the surface to form an ignitable mixture with air. It is a measure of fire hazard rather than of boiling behavior. Liquids with lower flash points present greater fire risk because ignitable vapors form at lower temperatures.

Question 29Hazard Recognition, Evaluation, and Control

Personal protective equipment is placed lowest in the hierarchy of controls mainly because it:

  1. Requires approval from a certified safety professional
  2. Costs more than engineering controls over time
  3. Leaves the hazard in place and depends on correct use Correct answer.
  4. Slows production more than other control methods

Explanation

PPE does not remove or reduce the hazard itself; it only places a barrier between the hazard and the worker. Its performance depends on proper selection, fit, maintenance, and consistent use by the worker. Controls that eliminate or contain the hazard are therefore preferred.

Question 30Hazard Recognition, Evaluation, and Control

In radiation protection, the ALARA principle requires that occupational exposures be:

  1. Held below the annual regulatory limit at all times
  2. Kept as low as reasonably achievable, with social and economic factors considered Correct answer.
  3. Reduced only when a regulatory limit has been exceeded
  4. Measured with a personal dosimeter every work shift

Explanation

ALARA stands for as low as reasonably achievable and calls for reducing doses below regulatory limits wherever practical. The principle explicitly weighs the effort and cost of further reduction against the benefit obtained. It is a continuous improvement obligation rather than a single fixed ceiling.

Question 31Hazard Recognition, Evaluation, and Control

Which cold-related condition results from actual freezing of body tissue?

  1. Frostbite Correct answer.
  2. Chilblains
  3. Hypothermia
  4. Immersion foot

Explanation

Frostbite occurs when tissue, most often in the fingers, toes, ears, or face, freezes and ice crystals form in the cells. Hypothermia is a drop in core body temperature and can occur without any tissue freezing. Immersion foot and chilblains are nonfreezing cold injuries.

Question 32Hazard Recognition, Evaluation, and Control

Guards and devices on a mechanical power press are primarily installed to protect workers from hazards at the:

  1. Drive belt
  2. Flywheel
  3. Control panel
  4. Point of operation Correct answer.

Explanation

The point of operation is where the work is positioned and the press actually performs its cutting, punching, or shaping, so hands are at greatest risk there. Guards and presence-sensing devices keep body parts out of that danger zone during the stroke. Flywheels and belts are transmission hazards addressed by different guarding.

Question 33Hazard Recognition, Evaluation, and Control

A red-bordered label pictogram showing a skull and crossbones indicates which hazard?

  1. Carcinogenicity
  2. Corrosive to metals
  3. Oxidizing potential
  4. Acute toxicity Correct answer.

Explanation

Under the globally harmonized system, the skull and crossbones pictogram identifies substances with acute toxicity that can be fatal or toxic if swallowed, inhaled, or brought into contact with skin. Carcinogenicity uses the health hazard pictogram showing a human silhouette with a starburst on the chest. Oxidizers use a flame over a circle, and corrosives use a pictogram showing damage to a hand and a surface.

Question 34Hazard Recognition, Evaluation, and Control

In industrial hygiene, the immediately dangerous to life or health value for a substance represents the concentration that:

  1. Represents the odor threshold for most workers
  2. Is expected to cause discomfort within one hour
  3. Is the legal limit for an eight-hour work shift
  4. Poses an immediate threat of severe health effects or escape impairment Correct answer.

Explanation

An IDLH value describes an atmospheric concentration that could cause death, irreversible health effects, or immediate eye or respiratory irritation, or that would impair a worker's ability to escape. It is used to select respiratory protection, since atmosphere-supplying respirators are required in IDLH conditions. It is not an exposure limit for routine work.

Question 35Hazard Recognition, Evaluation, and Control

Which statement correctly distinguishes OSHA permissible exposure limits from ACGIH threshold limit values?

  1. PELs are enforceable legal limits; TLVs are nonregulatory recommendations Correct answer.
  2. PELs apply only to airborne chemicals; TLVs apply only to noise
  3. PELs are advisory; TLVs carry the force of law
  4. Both are enforceable, but TLVs apply only to construction work

Explanation

PELs are adopted through regulation and are legally enforceable by OSHA, while TLVs are consensus guidelines published by a professional body. Many safety programs use the more current TLVs as internal targets even where they are not law. Both types of value address a wide range of chemical and physical agents.

Question 36Hazard Recognition, Evaluation, and Control

A ceiling limit for an airborne contaminant specifies that the concentration:

  1. Must never be exceeded at any point during the work shift Correct answer.
  2. Applies only to workers wearing respirators
  3. May be exceeded for up to 15 minutes per shift
  4. May be averaged over an eight-hour shift

Explanation

A ceiling limit is an absolute upper bound that should not be exceeded even momentarily. It is used for fast-acting substances whose effects depend on peak concentration rather than on an eight-hour average. Short-term exposure limits differ because they permit a defined excursion when averaged over 15 minutes.

Question 37Hazard Recognition, Evaluation, and Control

A skin notation placed next to an exposure limit indicates that:

  1. The substance causes skin burns on direct contact
  2. The substance is safe to handle without gloves
  3. Air monitoring alone is adequate to assess exposure
  4. Dermal absorption can contribute substantially to total body burden Correct answer.

Explanation

The notation warns that the substance can be absorbed through intact skin in amounts that add meaningfully to the total dose. For such agents, air sampling alone may underestimate exposure, so skin contact must be prevented by engineering controls and appropriate gloves. The notation does not describe corrosivity, although some skin-notation substances are also corrosive.

Question 38Hazard Recognition, Evaluation, and Control

Under the 5 dB exchange rate used by OSHA, an exposure at 95 dBA is permitted for how long before the daily noise dose limit is reached?

  1. 8 hours
  2. 2 hours
  3. 6 hours
  4. 4 hours Correct answer.

Explanation

With a 5 dB exchange rate, each 5 dB increase halves the permitted exposure duration. The baseline is 90 dBA for 8 hours, so 95 dBA is allowed for 4 hours and 100 dBA for 2 hours. Dose accumulates according to the ratio of actual time to permitted time at each sound level.

Question 39Hazard Recognition, Evaluation, and Control

A worker is exposed to 100 dBA for 1 hour. Using the OSHA 5 dB exchange rate, this exposure represents what fraction of the daily noise dose?

  1. 75 percent
  2. 50 percent Correct answer.
  3. 100 percent
  4. 25 percent

Explanation

At 100 dBA the permitted duration is 2 hours, obtained by halving the 8-hour allowance twice from 90 dBA. One hour of exposure is therefore one half of the permitted duration, giving 50 percent of the daily dose. The dose is calculated as the ratio of exposure time to permitted time.

Question 40Hazard Recognition, Evaluation, and Control

Which practice best prevents heat illness in a new worker starting a job in a hot environment?

  1. Requiring the worker to wear vapor-impermeable clothing
  2. Providing salt tablets with every meal
  3. Gradually increasing exposure time over one to two weeks Correct answer.
  4. Assigning the worker to the hottest task on the first day

Explanation

Acclimatization is developed by progressively increasing heat exposure over roughly one to two weeks, allowing the body to increase sweating and stabilize cardiovascular response. New and returning workers are at highest risk during this period. Salt tablets and impermeable clothing do not confer acclimatization and may add risk.

Question 41Hazard Recognition, Evaluation, and Control

A ground-fault circuit interrupter used for temporary power on a construction site is designed to open the circuit at a current imbalance of about:

  1. 500 milliamps
  2. 50 milliamps
  3. 5 milliamps Correct answer.
  4. 1 amp

Explanation

GFCIs sense small differences between current leaving and returning and trip at roughly 5 milliamps, a level intended to prevent ventricular fibrillation in a person contacting an energized conductor. They act within a fraction of a second after detecting the imbalance. Higher trip levels would not provide the intended shock protection.

Question 42Hazard Recognition, Evaluation, and Control

Under OSHA fall protection requirements, a personal fall arrest anchorage must be capable of supporting at least:

  1. 10,000 pounds per employee
  2. 1,000 pounds per employee
  3. 500 pounds per employee
  4. 5,000 pounds per employee Correct answer.

Explanation

Anchorage points for personal fall arrest systems must support at least 5,000 pounds per attached employee, or be designed with a safety factor of at least two under the supervision of a qualified person. This capacity accounts for the dynamic forces generated when a fall is arrested. The 5,000-pound figure is a strength criterion, not the worker's body weight.

Question 43Hazard Recognition, Evaluation, and Control

Which task belongs to the attendant stationed outside a permit-required confined space?

  1. Performing the initial atmospheric testing inside the space
  2. Entering the space to rescue a worker without additional help
  3. Closing the entry permit before the work has finished
  4. Keeping an accurate count of entrants and summoning rescue Correct answer.

Explanation

The attendant remains outside, tracks who is in the space, maintains communication, and calls for rescue without entering. Attendants are explicitly prohibited from entering to attempt rescue unless they are trained and equipped and are relieved by another attendant. Atmospheric testing before entry is an entrant or test director function, not the attendant's monitoring duty.

Question 44Hazard Recognition, Evaluation, and Control

A gas has a lower flammable limit of 4 percent and an upper flammable limit of 16 percent by volume in air. Which concentration is capable of burning?

  1. 10 percent Correct answer.
  2. 20 percent
  3. 2 percent
  4. 18 percent

Explanation

Only concentrations between the lower and upper flammable limits can support combustion, so any value between 4 and 16 percent is within the flammable range. Below the lower limit the mixture is too lean, and above the upper limit it is too rich to burn. A combustible gas meter alarms as concentrations approach the lower limit because that is where ignition becomes possible.

Question 45Hazard Recognition, Evaluation, and Control

Under OSHA respiratory protection requirements, tight-fitting respirators must be fit tested:

  1. Only when the worker requests it
  2. Before initial use and at least annually thereafter Correct answer.
  3. Once every three years
  4. Only after a failed medical evaluation

Explanation

Fit testing is required before the respirator is first used and at least once a year afterward, and whenever a different size, model, or style is introduced. Annual retesting confirms that the facepiece still seals adequately as facial features and equipment change. Fit testing is separate from, and does not replace, the medical evaluation.

Question 46Hazard Recognition, Evaluation, and Control

A worker is exposed for a full 8-hour shift to 25 ppm of substance A, whose permissible exposure limit is 50 ppm, and 25 ppm of substance B, whose limit is 25 ppm. Applying the additive formula for mixtures, the combined exposure is:

  1. Acceptable, because each substance is at or below half of its individual limit
  2. Acceptable, because the two substances have different exposure limits
  3. Unacceptable, because the sum of the exposure ratios is 1.5 Correct answer.
  4. Unacceptable, because the sum of the exposure ratios is 0.75

Explanation

For substances with similar health effects, exposures are combined by summing each concentration divided by its limit. Here 25/50 plus 25/25 equals 0.5 plus 1.0, or 1.5, which exceeds 1 and indicates an overexposure despite each individual measurement appearing permissible. Substance B alone already equals its limit, so any additional additive burden pushes the mixture over the acceptable value.

Question 47Hazard Recognition, Evaluation, and Control

What is the approximate concentration in milligrams per cubic meter of 50 ppm of toluene, which has a molecular weight of 92, at 25 degrees Celsius and 1 atmosphere?

  1. 94 milligrams per cubic meter
  2. 188 milligrams per cubic meter Correct answer.
  3. 1,840 milligrams per cubic meter
  4. 460 milligrams per cubic meter

Explanation

The conversion at 25 degrees Celsius is milligrams per cubic meter equals ppm multiplied by molecular weight divided by 24.45. Multiplying 50 by 92 gives 4,600, and dividing by 24.45 yields about 188. The divisor changes with temperature and pressure because it represents the molar volume of an ideal gas.

Question 48Hazard Recognition, Evaluation, and Control

Which hazard analysis technique begins with a defined top event and works backward to identify combinations of equipment and human failures that could produce it?

  1. Hazard and operability study
  2. What-if analysis
  3. Job hazard analysis
  4. Fault tree analysis Correct answer.

Explanation

Fault tree analysis is a deductive method that starts from an undesired top event and uses logic gates to trace the combinations of basic failures that could cause it. This structure supports quantification of the probability of the top event. The other methods are inductive, beginning with the system or task and asking what could go wrong.

Question 49Hazard Recognition, Evaluation, and Control

Under OSHA hearing conservation requirements, a standard threshold shift is defined as an average change in hearing threshold of at least:

  1. 5 dB at 4000 and 6000 Hz
  2. 15 dB at 500 and 1000 Hz
  3. 25 dB at 500, 1000, and 2000 Hz
  4. 10 dB at 2000, 3000, and 4000 Hz Correct answer.

Explanation

A standard threshold shift is an average shift of 10 dB or more in either ear at the frequencies of 2000, 3000, and 4000 hertz. These frequencies are used because noise-induced hearing loss typically appears first in this range. Identifying a shift triggers notification, reevaluation, and follow-up actions under the hearing conservation program.

Question 50Hazard Recognition, Evaluation, and Control

To document compliance with an 8-hour time-weighted average permissible exposure limit, an industrial hygienist should collect:

  1. One sample from a fixed location in the center of the room
  2. Full-period consecutive samples covering the entire shift Correct answer.
  3. Samples only from workers who report symptoms
  4. A single 15-minute grab sample taken during the peak task

Explanation

A time-weighted average describes exposure across the whole shift, so the sampling must represent all tasks and periods, typically through consecutive full-period samples in the breathing zone. Short grab samples can characterize peaks but cannot alone demonstrate an 8-hour average. Fixed-area samples do not represent personal exposure and are used for other purposes.