1. CHAPTER 5
Occupational Exposure Limits and Assessment of Workplace Chemical Risks

2. Learning Objectives
Describe how occupational exposure limits (OELs) are used in workplace risk assessment.
Identify the key components of an OEL.
Identify the OELs created by various organizations.
Outline the process of OEL derivation.
Apply available information to working conditions to assess hazardous exposure levels.

3. Role of OELs In Workplace Risk Assessment
Hazard characterization
Systematic process for weighing the complete data to identify human health effects caused by exposure to a specific scenario
Dose-response assessment
Evaluation of the changes in incidence and severity of effects with increasing exposure (or dose)
Identify a quantitative dose for adverse effects that serves as a starting point for derivation of the OEL
OELs are intended to provide a quantitative estimate of the maximum air concentration that is believed to be safe for an occupational population exposed daily for a working lifetime

4. Role of OELs In Workplace Risk Assessment
Exposure assessment
Evaluation of the magnitude, duration, and frequency of exposures of a population of interest
Risk characterization
Integration of the OEL derived from a dose-response assessment and the exposure assessment that supports the level of risk assumed by the employer/agency

5. Importance of OELs
Provide a scientific basis to evaluate existing exposure control technologies and to identify worker health and medical surveillance needs
Promote risk communication by informing workers of potentially adverse health effects of chemical exposure
Provide a benchmark for exposure assessment comparisons

7. OEL Elements Concentration units
Recommended Time Weighted Average (TWA)
Additional notations

8. OEL Elements - Example

10. OEL Concentration Units
Most OELs are for inhalation exposures
Common unit of measure is the chemical’s mass per unit of air volume
Milligrams per cubic meter (mg/m3)
For solid particulates and liquid aerosols
Also fibers per cubic centimeter (f/cc) for asbestos
OELs for gases and vapors are based on the molar ratio (number of chemical molecules per air molecule)
Parts per million (ppm) or parts per billion (ppb)
For gases and vapors (i.e., gases formed when liquids evaporate)

11. ppm to mg/m3 conversions
Most organizations that set OELs will give the exposure limit in both mg/m3 and ppm, but sometimes you have to do a conversion
ppm = (mg/m3 x 24.45) / molecular weight
Example:
Convert 398 ppm of isopropyl alcohol to mg/m3 (MW = 60.1 g/mol)
Convert 0.15 mg/m3 of ozone to ppm (MW = 48 g/mol)
Convert 1 ppm benzene to mg/m3 (MW = g/mol)

12. Time-Weighted Average (TWA)
Most OELs are given as:
Full-shift (8-hour or 10-hour) TWA
Short-term exposure limit (STEL)
Ceiling limit (C)
Full shift (8-hour or 10-hour) TWA assumes fluctuating exposures during the day
Intended to prevent chronic health effects that are dependent on the cumulative dose of the chemical that a worker receives over a working life time (40 hours/week for years)

13. Short-Term Exposure Limit (STEL)
A time-weighted average exposure limit whose averaging time is short
In most organizations, a STEL is a 15-minute TWA exposure limit, although other averaging times can be specified for a given chemical (i.e., 30 minutes for excursion limit of asbestos)
For chemicals that can generate health effects following short periods (minutes) of exposure

14. Ceiling Limit
A maximum exposure, or ceiling limit that should not be exceeded at any time during the workday
For chemicals with rapid or nearly immediate effects

15. Excursion Limits
What if there is not enough data to develop a STEL or C?
ACGIH approach for excursion limits:
Three times the value of the full-shift TWA
Provided five times the full-shift TWA is never exceeded
Assumes good emission controls

16. Hazard Notations
Additional factors may affect the interpretation or use of OELs, such as:
toxicity via skin absorption
potential for sensitization
carcinogenicity
Sensitizer: allergic reaction, become increasingly reactive to subsequent exposures to a chemical
SKIN notation = dermal exposure may be a significant course of systemic (absorbed) and toxic dose
Therefore, skin exposure should be limited + consider inhalation
DSEN notation = dermal sensitizer
RSEN notation = respiratory tract sensitizer

17. Biological Exposure Indices (BEIs)
Exposure limits based on the levels of chemicals in the body or excreta have been developed to measure the internal dose
These values are not fine lines between safe and dangerous concentrations and should not be used by anyone untrained in the discipline of industrial hygiene. 
BEI® determinants are an index of an individual’s “uptake” of a chemical(s).
Air monitoring to determine the TLV® indicates the potential inhalation “exposure” of an individual or group.

18. Biological Exposure Indices (BEIs)
The uptake within a workgroup may be different for each individual for a variety of reasons.
Physiological makeup and health status of worker
Work rate intensity and duration, skin exposure, temperature, humidity, exposure to other chemicals
Community and home air pollutants, smoking, alcohol/drug use
Most BEIs® are based on a direct correlation with the TLV® (i.e., the concentration of the determinant that can be expected when the airborne concentration is at the TLV®).
Some of the BEIs® (e.g., lead) are not derived from the TLV®, but directly relate to the development of an adverse health effect.

19. Fundamentals of OEL Derivation
Define the scenario and develop the problem formulation.
Gather and summarize the scientific literature.
Select a dose point that will protect workers from the chemical’s toxic effect.
Perform extrapolations to increase the relevance of the point of departure:
Adjust for route of exposure and exposure duration/patterns using default assumptions on rates of ingestion/inhalation or physiologically based pharmacokinetic (PBPK) models.
Perform animal-to-human extrapolations and human variability extrapolations
Apply any additional uncertainty factors
Submit the value for external review.

20. No Observable Adverse Effect Level (NOAEL)
The highest dose that does not cause an adverse effect

21. Lowest Observable Adverse Effect Level (LOAEL)
The lowest dose that causes an adverse effect

22. Point of Departure (POD)
The NOAEL or LOAEL that best identifies the boundary of the onset of adverse effects is selected for OEL derivation
An estimation of “safe dose”

25. Example A typical OEL data set assessment:
Rats were exposed 6 hours per day, 5 days per week to 0, 10, 25, or 50 ppm solvent for 2 years
No effects were observed at 10 ppm
Signs of liver toxicity observed at 25 ppm and above
No significant effects data in humans are available
No studies of reproductive effects are available
The chemical has moderately acute toxicity but is not genotoxic or a sensitizer
OEL derivation = (Point of Departure) / (Uncertainty Factors)
OEL = (10 ppm) / (3 UFA x 3 UFH x 1 UFL x 1 UFS x 3 UFD) = 0.4
Therefore OEL = 0.3 as a full shift TWA with no hazard notations

26. OEL Use and Interpretation
OELs are one of the require inputs to support decisions about risk
Most common approach is to develop a hazard quotient
Hazard quotient = HQ
HQ is the ratio of measured or estimated exposure : OEL
HQ = exposure / OEL
Value of 1 means the exposure is equal to the OEL for the exposure scenario
Higher HQ means higher likelihood of adverse effects
Many organizations requires exposures with an HQ >1 must initiate control measures
The HQ concept is the most common risk characterization method

28. Action Limits from OSHA
Some OSHA substance-specific standards include an action limit to spur medical surveillance and exposure monitoring
Noise
Lead
Acrylonitrile
Others

29. Margin of Safety
In this concept, the OEL and exposure limit are reversed, and the goal is a high margin of safety
OEL much greater than the level of exposure
Qualitative like HQ method

30. Considerations in Using OELs
Use and evaluate them based on their uncertainties
Derived using UF that can reflect orders-of-magnitude differences in judgement
Variability due to data differences, method differences, and risk tolerance differences
Precision = varies minimally from a defined standard
OELs are not definitive limits between “safe” and “dangerous” therefore not “precise”
Accuracy = exact conformity to fact
OELs are estimates of a concentration that is safe
OELs are not precise, but they are often accurate  serve as a best estimate of safe concentration

31. OEL Resources
Many organizations develop OELs and the detailed documentation that describes the basis of the OEL
However, no quantitative health-based exposure guidance is available for the majority of chemicals encountered in the workplace
Use a group of OEL-related tools to support quantitative risk characterization
Use hierarchy of OELs approach
More toxicological and epidemiological data available  move higher up the hierarchy

33. OSHA and OELs
Including sample calculations from OSHA guidance

34. OSHA Definitions for Exposure Limits
"C"-Ceiling Values. An employee's exposure to any substance in Table Z-1, the exposure limit of which is preceded by a "C", shall at no time exceed the exposure limit given for that substance.
If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time during the working day.
8-hour time weighted averages. An employee's exposure to any substance listed in Table Z-1/Z-2, in any 8-hour work shift of a 40-hour work week, shall not exceed the 8-hour time weighted average limit given for that substance in Table Z-1/Z-2.

35. OSHA Definitions for Exposure Limits
The cumulative exposure for an 8-hour work shift shall be computed as follows: E = (Ca Ta + Cb Tb+. . .Cn Tn) ÷ 8 Where: E is the equivalent exposure for the working shift. C is the concentration during any period of time T where the concentration remains constant. T is the duration in hours of the exposure at the concentration C. The value of E shall not exceed the 8-hour time weighted average specified in subpart Z of 29 CFR Part 1910 for the substance involved.

36. OSHA Definitions for Exposure Limits
In case of a mixture of air contaminants an employer shall compute the equivalent exposure as follows: Em= (C1÷L1 + C2÷L2) (Cn ÷ Ln) Where: Em is the equivalent exposure for the mixture. C is the concentration of a particular contaminant. L is the exposure limit for that substance specified in Subpart Z of 29 CFR part The value of Em shall not exceed unity (1).

37. Limitations of all OELs
Regulatory limits are the product of toxicologic and economic compromises – not exposure boundaries that protect all workers.
Voluntary OELs are recommended as guidelines – not a fine line between “safe” and “unsafe.”
Calculations are used to estimate the likely exposure for a worker given certain engineering and administrative controls.

38. Limitations of all OELs
Common recognized problems with OELs:
Difficult to acquire a truly representative breathing zone sample.
Uncertainty about the extent of absorption of the amount inhaled.
Non-routine or repetitive work – air samples characterize work operations only on the day the sample is taken.
Variations in particle size, absorption, and particle solubility.
Accidental or deliberate contamination of a sample.

39. Mixtures
An additive effect is one in which the combined health effect of the simultaneous exposures is equal to the sum of the effects of each individual substance alone.
For example, the cholinesterase inhibition of two organophosphate pesticides is usually additive when exposure occurs together.
Similarly, many solvents have narcotic effects that are considered additive in nature.
Below are additional examples of chemicals which have additive effects when exposure occurs together:
acetonitrile + cyanides
carbon monoxide + methylene chloride

40. Mixtures
A synergistic effect is one in which the combined effect of the exposures is much greater than the sum of the individual effects.
Classic examples include the synergistic effect of carbon tetrachloride and ethanol on liver toxicity and the synergistic effect on the lungs of smoking and exposure to asbestos.

41. Mixtures
Potentiation describes a condition in which the target organ toxicity of a particular chemical is markedly increased by exposure to another chemical which does not ordinarily have toxic effects on that organ or system.
For example, isopropanol is not a liver toxin, but when combined exposure to isopropanol and carbon tetrachloride (liver toxin) occurs, the liver toxicity is much greater than that due to carbon tetrachloride alone.
Ethanol potentiates the toxicity of many other chlorinated hydrocarbons.

42. Mixtures
Antagonism refers to the situation in which the toxic effects of two chemicals interfere with each other, or the effects of one chemical are actually reduced by exposure to another chemical. 
This is the basis for many antidotes. 

43. Sample Calculations!
To assist you in completing Assignment #2…

44. Calculations: TWA
Assume that “Substance A” has an 8-hour TWA limit of 100 ppm according to OSHA in Table Z-1.
Assume that an employee is subject to the following exposures:
Time
Exposure
2 hours
150 ppm
75 ppm
4 hours
50 ppm

45. Calculations: TWA Answer
[(2 × 150) + (2 × 75) + (4 × 50)] ÷ 8 = ppm
Since ppm is less than 100 ppm, the 8-hour time weighted average limit, the exposure for Substance A is acceptable.
Time
Exposure
2 hours
150 ppm
75 ppm
4 hours
50 ppm

46. Calculations: Mixtures
You conducted sampling and the laboratory analysis provided the following data:
Substance
Actual concentration of 8-hour exposure (ppm)
8-hour TWA PEL (ppm) B
500
1,000 C 45
200 D 40
If the equivalent exposure (OELm) is <1, it is considered to be an acceptable exposure.

47. Calculations: Mixtures Answer
Substituting in the formula, we have:
OELm = (500 ÷ 1,000) + (45 ÷ 200) + (40 ÷ 200)
OELm =
OELm = 0.925
Since OELm is less than unity (1), the exposure combination is within acceptable limits.
Substance
Actual concentration of 8-hour exposure (ppm)
8-hour TWA PEL (ppm) B
500
1,000 C 45
200 D 40

48. Calculations: Mixtures Part 2
What if the ACGIH TLV® TWAs are as shown in the table?
Is this still an acceptable exposure?
Substance
Actual concentration of 8-hour exposure (ppm)
ACGIH
8-hour TLV-TWA (ppm) B
500
750 C 45
150 D 40
100

49. Calculations: Mixtures Part 2 Ans.
OELm = (500 ÷ 750) + (45 ÷ 150) + (40 ÷ 100)
OELm =
OELm = 1.367
Since OELm is more than unity (1), the exposure combination is NOT within acceptable limits.
Substance
Actual concentration of 8-hour exposure (ppm)
ACGIH
8-hour TLV-TWA (ppm) B
500
750 C 45
150 D 40
100

50. Calculations: Min. Sample Time
Minimum sample time (MST) = minimum sample volume (MSV) / flow or sampling rate (SR)
For Substance E, the minimum sample volume is 240 liters, and the flow rate is 2 liters per minute (L/min).
How many samples should you take to represent an 8-hour day?

51. Calculations: Min. Sample Time Ans.
The sampler could be changed out after 2-hours, and full-shift sampling could be conducted using four 2-hour time segments.
However, if the minimum sample volume is 600 liters and the flow rate is 2 L/min, a four-hour sample would be insufficient.

52. Calculations: PPM to mg/m3
You conducted air sampling for phenol and the laboratory gave you the results in mg/m3. The PEL is given in ppm. The molecular weight of phenol is g/mol. What is the exposure in ppm?
Given: 12.5 mg/m3 phenol

53. Calculations: PPM to mg/m3 Answer
ppm = (12.5 mg/m3 x 24.45) / 94.11
Answer = 3.25 ppm
Is this above or below the OSHA PEL?
Below (5 ppm)
Is this above or below the ACGIH TLV?

54. Calculations: Air Volume
Flow rate × time = air volume
Example: An IH sampled for 100 minutes at a flow rate of 0.2 L/min, what is the air volume?
0.2 L/min × 100 min = 20 L

55. Sources of Toxicological Information
Safety Data Sheets
TOXNET
ATSDR

56. Safety Data Sheets Section 8: Exposure Controls / Personal Protection
Section 11: Toxicological Information

57. TOXNET

58. ATSDR

59. Summary
Toxicity is used to describe the ability of a substance to have an adverse effect on the health or well-being of a human.
Whether ill effects occur depends on:
Properties of chemical, dose, route of exposure, and susceptibility/resistance of exposed individual
Four routes of entry:
Inhalation, skin absorption, ingestion, injection
Response depends on the dose received and the time interval during which the dose is administered.

60. Presentation Sources
Part of the presentation materials were retrieved from:
Federal Occupational Health and Safety Administration
American Industrial Hygiene Association (AIHA)
Industrial Hygiene Reference and Study Guide, 2nd Edition, from AIHA
OSHA Technical Manual, “Personal Sampling for Air Contaminants”
ACGIH TLVs® and BEIs® (2012)