1. Pollution Prevention & Environmental Essentials Conference
Paul Haas CSP, CIH
University of South Florida
SafetyFlorida Consultation Program

2. Emergency Planning and Disaster Control
Safety and Health Planning for the Trades

3. Exposure Modeling (Spill Models)
Exposure Assessment Using Modeling To Determine Air Concentration After Chemical Releases

4. Exposure Modeling (Spill Models)
The use of models to describe employee exposures is not new, but the Occupational Safety and Health Administration (OSHA) has proposed a simple methodology to use for calculation of air concentrations from spills.
OSHA has not determined if these will apply in all cases where employees may be exposed.
Chemical reactivity is not considered in these models.

5. Exposure Modeling (Spill Models)
More than 20 chemical accidents are reported each day in the U.S, according to data collected by the U.S. Environmental Protection Agency. Responding to these accidents is a dangerous but essential job. In the U.S., this job is usually handled by firefighters from local fire departments.

6. Exposure Modeling (Spill Models)
RIO NEUQUEN chemical incident, Houston, Texas, July One of many containers of the culprit substance is pictured.
Models can be used for release of chemicals.

7. Constant Decay Model

8. EXPOSURE MODELING Principles from physical chemistry are applied
Applications will be presented in examples
Graphical presentations from Dr. Mark Nicas – UC Berkeley are provided

9. Elements of the Exposure Model
A chemical substance release
Determine the air concentration from a release into a room
And
Estimate if the release may pose a probable risk inhalation health hazard.

10. Elements of the Exposure Model
An airborne exposure model uses the following elements:*
The contaminant mass emission rate
The contaminant dispersion in a room
The employee exposure pattern
*Source – OSHA TECHNICAL MANUAL ON PHYSICAL – CHEMICAL MATHEMATICAL EXPOSURE MODELS

11. EXPOSURE ASSESSMENT
When must an employer conduct an exposure assessment?
When there is a substance specific standard (e.g. lead, methylene chloride)
When employees notice symptoms or complain of respiratory effects
When the workplace contains visible emissions (e.g. fumes, dust aerosols)

12. EXPOSURE ASSESSMENT
OSHA Regulations for methods to determine employee exposure can be found for the following:
HAZWOPER – 29 CFR
RESPIRATORS – 29 CFR
SUBSTANCE SPECIFIC STANDARDS – 29 CFR – 1052 (e.g. Formaldehyde)

13. EXPOSURE ASSESSMENT
OSHA Regulations do not specify how the employer is to make a reasonable estimation for the purposes of selecting respirators for example (osha.gov/SLTC/respiratory_advisor)
OSHA Substance Specific Standards allow for the use of ‘objective evidence’ to estimate exposure.

14. EXPOSURE ASSESSMENT
When? What? How much employee exposure is there in the workplace?
Sampling – Personal exposure monitoring
Objective information – Data
Variation – Sampling + Data + Safety Factors

15. Air Monitoring Equipment
What To Do:
1. N/A
Discussion:
1. Air monitoring equipment should be employed whenever work processes or response processes change. Other areas where air monitoring are important are:
a) change in job task(s)
b) changes in weather or temperature
c) anticipated changes in ambient air levels of
contaminants
2. Air monitoring can be either active, with meters or similar devices, or passive using badges.

16. Personal Samplers What To Do: 1. N/A Discussion:
1. For personal sampling, should be accomplished using the “Gold” standard and it is the best method to determine an individual’s exposure.

17. Personal Samplers- Media
What To Do:
1. N/A
Discussion:
1.Various sample media are used to collect mists, gases and vapors such as these shown on the slide.

18. Other Types Site Monitoringof
Site Monitoring
HAZCAT Kit
Geiger Counter
- radiation
Specialty Monitors
- Passive monitoring badges
- TIFF 5000
What To Do:
1. N/A
Discussion:
1. There are other types of equipment that can be used to conduct site assessment and monitoring. A few examples are:
HAZCAT Kit -Identifies or categorizes over 1,000 hazardous and non-hazardous substances in 3 to 9 steps or approximately 10 minutes Contains everything needed for efficient field testing of both liquids and solids.
Geiger Counter - used to determine if there is a radiation hazard
Passive Monitoring Devices - there are various types on the market such as organic vapor and mercury
TIFF “sniffs” the air for Halogenated organic materials

19. EXPOSURE ASSESSMENT BASIC TERMS Exposure Model Air Contaminants
Parts Per Million
Milligrams Per Cubic Meter
Employee Exposure

20. EXPOSURE ASSESSMENT BASIC TERMS Exposure Model
An exposure model is the description of a:
Air contaminant
Room or space volume
Employee exposure

21. EXPOSURE ASSESSMENT BASIC TERMS Air Contaminants
Parts Per Million (PPM)
PPM is a ‘dimensionless number’
A 1 PPM concentration is $1 in a $1,000,000
Milligrams Per Cubic Meter
Is in weight per unit volume
Expressed in milligrams per cubic meter for gases, mists, vapors

22. EXPOSURE ASSESSMENT BASIC TERMS Employee Exposure
Who, What, When, Why, How Exposed?
‘Typical’ or ‘Emergency’ release
What is an ‘Incidental’ release as defined in the HAZWOPER regulation?

23. Chemical Identity and Form
·        If the chemical is a gas – The molecular weight and the gas density
·        If the chemical is a liquid – The molecular weight and the vapor pressure
·        If the chemical is a solid – The molecular weight

24. Material Release Parameters
·        The room or space volume (V) in cubic meters (m3)
·        The room supply/exhaust air rate (Q) in cubic meters per minute (m3/min)
·        The contaminant emission rate function (G) in milligrams per minute (mg/min)

25. Room Ventilation and Volume
V = Room volume determined by (Length X Width X Height)
Q = Air supply. It is assumed to be the room’s entire supply/exhaust air exchange rate from a mechanically – driven system. note: If room air supply is not known use the following assumptions**
**Air speed (s) = m/min in a room with no strong air motion
Air speed (s) = 7.6 m/min in a room with strong air currents

26. Mass Emission
Q = The product of the air speed times the room area (Speed X Length X Width)
Gt = The emission rate function (Gt) is expressed in a release rate of mass-per-time or milligrams per minute (mg/min).
Air Concentration in a room after a release (C0) is a ‘worst case’ scenario of the Emission Rate Function over the Room Exhaust Rate or (Gt/Q)

27. ‘Worst Case Scenerio’
The air concentration (C0) of a release of a material in a room using the Exposure Model Gt/Q is a ‘worst case’ scenario model
Use the ideal gas law equation – (PV/nRT) to determine the concentration C0

28. ‘Worst Case Scenerio’ The chemical is continually exposed to room air
There is no initial air dispersal
Room temperature is constant
There is sufficient time to reach equilibrium
Enough chemical mass exists
The ideal gas law holds

29. Ideal Gas Law Background
A variation of Boyle’s and Dalton’s laws
P1 V1 = P2 V2
T T2
PTOTAL = P1 + P2 + … + Pk, for k constituents
Application includes converting a mass of liquid evaporating per minute to the vapor volume evaporating per minute

30. Dalton’s Law
The total pressure of a gaseous mixture is the sum of the partial pressures exerted by each constituent of the mixture
PTOTAL = P1 + P2 + … + Pk, for k constituents
According to the ideal gas law, the mole fraction (Yi) of a gas constituent is
Yi = Pi / PTOTAL , expressed in ppm (parts per million)

31. Ideal Gas Law Constants
P = Pressure in mm Hg
V = Volume in M3
T = Temperature in K
R = Gas Constant
0.623 mm Hg M3MOL-1K-1
n = number of moles of gas

32. Ideal Gas Law
PV = nRT
At NTP (298.3 K and 760 mm Hg), one mole of gas generates a gas volume V = M3 (24.45 Liters)
What about gas in containers?
One mole of gas introduced into a rigid container = ? V

33. Ideal Gas Law
One mole of gas introduced into a 1 M3 container will occupy 1 M3 not M3
However, a constant temperature will yield a gas partial pressure of 18.6 mm
P = nRT/V = (1 mol)(0.623 mm Hg M3MOL-1K-1)(298.3 k)/1 M3 = 18.6 mm

34. G(t), M3/min = (G(t), mg/min)(0.001 g/mg)
Vapor Volume
Mass per time is converted to volume per time using the following equation:
G(t), M3/min = (G(t), mg/min)(0.001 g/mg) X
RTA/Mol. Wt. PA
This equation will be explained in examples to illustrate the gas conversion relationship

35. Vapor Pressure (Eq)
When the rate of evaporation = rate of condensation in the headspace of containers
If the system is at equilibrium and the headspace air is saturated with chemical vapor
The partial pressure (Pv) of a chemical is related to the temperature, e.g.
Pv of benzene at 20 C is 75 mm Hg and 96 mm Hg at 25 C

36. Saturation Concentration (Csat)
Csat in ppm
= PV in mm Hg X 106
760 mm Hg
This unifies Boyle’s and Dalton’s Law into the ideal gas law

37. Saturation Concentration (Csat)
Csat in mg/M3
= (Csat in ppm) X Mol. Wt.
24.45
This is expressed at NTP conditions (298.3 K, 760 mm Hg)

38. Saturation Concentration (Csat)
Csat in mg/M3
Can also be determined by the following:
= PV X Mol. Wt. X 1000 RT
This is the product of the vapor pressure of n moles of gas and the chemical’s molecular weight

39. Spill Modeling
Example 1 – Estimate the air concentration using a ‘worse case scenario using an identified chemical and mass emission rate
Example 2 – Determine the air concentration of a spill after one minute knowing the room volume and dispersal rate

40. Example #1 (C2Cl4 Release)
A container of perchloroethylene (C2Cl4) is left open in a unventilated cabinet.
An individual opens the door and is exposed
What is the perchloroethylene concentration in ppm that the employee is exposed to?

41. Example (C2Cl4 Release) Use the following values in the Csat equation:
T = 20 C; Perchloroethylene PV = 14 mm Hg at 20 C; Cabinet Pressure = 760 mm Hg
Csat = 14 mm Hg X 106
760 mm Hg
Csat = 18,421 ppm

42. Dalton’s Law (Remember!!!)
To be rigorous, one must add the partial pressure of the perchloroethylene to the total pressure in the cabinet.
This yields a Csat of 18,090 ppm (A 2% difference)
This estimate is based on a Qintial of 0, any air movement (Q) > 0 would revise the estimate of concentration downward.

43. Exposure Limits for C2Cl4
For perchloroethylene (C2Cl4 ) the acceptable OSHA maximum peak is 300 ppm*
* 29 CFR , Table Z-2
The initial exposure concentration of 18,090 ppm C2Cl4 is 60 times greater than the acceptable ceiling limit.

44. Example #2 (Gas Release)
Carbon Dioxide (CO2) is released into a 10 X 20 square meter (33 X 65 foot2) room from a 10 - liter container. How many parts per million (ppm) of CO2 are released?
Assume that the air speed in the room is 3 meters/minute.

45. Solution
First, determine the molecular weight of Carbon Dioxide (CO2) in a molar unit (mole) of gas. It is the product of the gas molecules of carbon (C) and oxygen (O):
C + O + O2 = or
44 milligrams CO2

46. Solution
Then, assume that carbon dioxide gas occupies all of the container and all of the gas is released at once. The total volume of the container is 10 liters (L). The amount of gas released in cubic meters is as follows:
10 liters X cubic meter (M3)/ 1000 liters or 0.01 m3

47. Solution
In a ‘normal’ temperature (T) and pressure (P) environment, a mole of gas occupies a volume (V) 0f M3 (24.45 L)
 The number (n) of moles of CO2 released is:
0.01 M3 CO2 / M3/mole
= 0.4 moles CO2

48. Answer
To determine a ‘worst-case’ scenario, we first determine the partial pressure of the gas release in the room air.

49. Answer
The partial pressure of the CO2 released is determined using the ideal gas equation:
P = nRT/V

50. Temperature & Pressure
Room temperatures must be converted to degrees Kelvin (K) using the ideal gas law, so room temperature in C is added to 273.3
Pressure is expressed in milliliters of mercury (mm Hg)

51. Answer
PCO2 = nRT/V
P = (0.4 mole CO2)( mm Hg . m3 . mole-1 . K-1)(298.3 K)/ 0.01 M3
PCO2 = / 0.01 mm Hg = 2.55 mm Hg

52. Answer P1 nRT/V/ P2 nRT/V Partial Pressure = P1/ P2
Concentration is then expressed as the vapor volume of a chemical in its ratio to the vapor volume of air at atmospheric pressure (V/V).

53. V/V – vapor pressure of contaminant X 106
Answer
The V/V application model is a ‘best-guess’ assumption of exposure. This is the estimated vapor volume of a contaminant in a million parts of air (ppm).
V/V – vapor pressure of contaminant X 106
vapor pressure of air

54. Concentration (C0)
The ‘worst-case concentration (C0) of carbon dioxide released into the room is determined by the following equation:
(C0) = Partial Pressure/ Total Atmospheric Pressure in a million parts of air (ppm)

55. Concentration (C0)
(C0) = 2.5 mm Hg Carbon Dioxide / 760 mm Hg X 106 = X 106
(C0) is 3279 ppm of carbon dioxide from the release of the 10 liter container.

56. Concentration (Ct)
(Ct) is the concentration at a time from the initial release C0 to a time t
This takes into account diffusion of the release into a space by airflow 

57. Concentration (Ct)
What is the expected air concentration of CO2 in the room after one minute or (Gt/Q)?
Assume that in one minute the entire container is released or that 3279 ppm exists from the release. We need to determine the concentration of CO2 throughout the room.

58. Concentration (Ct)
The room floor area is 200 meters square and the air speed is assumed to be 3 meters/minute so Q is expected to be 3 M/min X 200 M or 600 cubic meters/ minute ( 600 M3/min)
Gt?

59. Concentration (Ct)
Convert 3279 ppm CO2 to milligrams per cubic meter (mg/M3)
C0 of carbon dioxide in mg/m3
= 3279 ppm X 44 mg/mole / M3/mole
= 5900 mg/M3 of carbon dioxide

60. Concentration (Ct)
Answer: After one minute there are mg/M3 / 600 M3 or 9.83 mg/ M3 of carbon dioxide remaining in the room.
The concentration is then reduced by a factor of 5900/10 or 59 times less

61. Constant Decay Theory by Dr. Mark Nicas
Applications:
Objective Exposure Assessment
Spill models from container filling
Confined Spaces?
Exponential decay model with an initial concentration (Co)

62. Constant Decay Theory by Dr. Mark Nicas
CIN -- mg/m V -- m3
G -- mg/min kL -- per min
Q -- m3/min C0 -- mg/m3

63. Constant Decay

64. Backpressure Effect on Contaminant Emission
Applications:
Low vapor pressure chemicals < 1 mm Hg
Pesticides
Nerve Agents
Net rate model with a steady state factor for chemicals with a low vapor pressure at equilibrium

65. Backpressure Effect on Contaminant Emission
Ct -- mg/m V -- m3
G0 -- mg/min t -- min
Q -- m3/min Csat -- mg/m3

66. Backpressure Effect on Contaminant Emission

67. NEAR / FAR FIELD (NF/ FF) with constant contaminant emission rate
Applications:
Operations with Dilution Ventilation
Assume a well mixed room dispersion pattern
Model describes zonal concentration in the near hemispherical free surface area and an air flow rate to a far field

68. Near Field / Far Field Model
G – mg /min Q -- m3/min
NF -- Volume m3  -- m3/min
FF -- Volume m3
 *( -b + SQRT((b2) - 4*cc))
Dr.Nicas

69. NF/ FF with constant contaminant emission rate

70. Spherical Turbulent Diffusion – Pulse Release
Applications:
Operations with Local Exhaust Ventilation
Storage tank filling operations in buildings
Models apply turbulent eddy diffusion for a continuous concentration near an emission source

71. The Spherical Turbulent Diffusion Model without Advection - Pulse Release
M0 -- mg Y -- m
DT -- m2/min X -- m
r -- SQRT(X2 + Y2 + Z2) -- m Z -- m

72. Spherical Turbulent Diffusion – Pulse Release