Exposure Bands. Relationship to the OEL (95. Rating

Transcription

1 Rules of Thumb 1

2 Exposure Unit The exposure assessment process is performed independently on each physical hazard, chemical or chemical mixture in the SEG. Each chemical - SEG combination is referred to as a exposure unit (EU). The product of the exposure assessment process is to classify exposures into ranges (exposure bands) as compared to an occupational exposure limit. 2

3 Exposure Rating Exposure Bands Relationship to the OEL (95 th Percentile) 0 X x Exposure Limit (OEL) x OEL < X x OEL x 0EL < X x OEL 3 05xOEL<X OEL 4 1 x OEL < X x OEL 5 2 x OEL < X x OEL 6 5 x OEL < X x OEL 7 10 x OEL < X x OEL 8 X 0.95 >50 x OEL 3

4 Types of Inhalation Hazards Vapors Gases Fumes Mists Aerosols Particulates Total Respirable fractions 4

5 Form and Conditions i Pure material Mixture Solubility Suspended Temperature Air Material Pressure 5

6 Mechanism for Introduction Evaporation Sublimation Decomposition Generation via chemical reaction Vaporization (heat to a point above boiling point - e.g., welding fume) Mechanically generated e.g., spraying or agitation 6

7 Vapor Pressure Vapor pressure is the pressureexerted by the gaseous phase of a two phase gas/liquid id or gas/solid system. The pressure of the vapor that is formed above its liquid id or solid is called the vapor pressure. If a substance is in an enclosed place the two phase system will arrive at an equilibrium state. This equilibrium state is a dynamic, balanced condition with no change of either phase. For a specific temperature, the pressure of the vapor measured at equilibrium state is the equilibrium or saturated vapor pressure. This pressure is a fraction of the total pressure, which is equal to 760 mm Hg at sea level. 7

8 Vapor Pressure An agents vapor pressure at it s boiling point is 760 mm of fh Hg (atmospheric pressure) The vapor of specific agents in mixtures is lower than the agent s vapor pressure in its pure state. Vapor pressure changes (increases) with temperature. Vapor pressures and boiling reports are usually reported on MSDS or are available in standard sources (e.g., TOXNET HSDB, NIOSH Pocket Guide) For comparison vapor pressures must use some comparable temperature t (usually 25 degrees C) 8

9 Exposure Potential Vapors: Rule-of-10 Level of Control Confined Space Virtually no circulation Poor Limited Circulation Good General ~ 6airturnovers/hr Capture Containment Fraction of Saturation 1/10 th of Saturation 1/ 100 th of Saturation 1/1,000 th of Saturation 1/10,000 th of Saturation 1/100, of Saturation 9

10 Example Benzene has a saturated vapor pressure at 25 C of 95.2 mm of Hg and atmospheric pressure is 760 mm of Hg. The saturated vapor pressure (VP) of benzene at 25 C is calculated in the following manner: Saturated VP of Benzene = (95.2 mm of Hg/760 mm of Hg) * 1,000, 000 = 125,000 ppm benzene If benzene were used in a room with only good general ventilation, exposures would be expected on the order of 125 ppm, a level e ~ 100 times its OEL. 10

11 Benzene Example Most IH s who have worked with exposure scenarios associated with benzene know that benzene requires containment t engineering i controls to reduce exposures to the order of magnitude of the 0.5 ppm TLV. 11

12 Vapor Pressure Changes with ihtemperature The vapor pressure increases with the temperature of the liquid rather than ambient temperature. Vapor pressure at temperatures other than ambient (25 degrees C) can be found in Standard chemistry texts Calculated using Antoine s Equation Clausius-Clapeyron Equation Estimated with a rule-of-thumb 12

13 Calculate Vapor Pressure Using Antoine s Equation log 10 (p) = A - (B / (t + C)) Where: p = vapor pressure (mm of Hg) t = temperature (C 0 ) A, B & C are constants unique to each chemical 13

14 Clausius-Clapeyron Equation Plot Log 10 (p) vs. 1/T Log 10 (p) = m(1/t) + C Where: p = vapor pressure in mm of Hg T = temperature in degrees Absolute (K) m = slope of the line c = intercept of the line Note: At the BP, the VP=760. If the vapor pressure of one more point is known, the vapor pressure at any temperature can be calculated 14

15 Estimated of Vapor Pressure with a rule-of-thumb The vapor pressure (VP) of the chemical will approximate double with each 10 C increase in temperature t of the liquid. id For example, if the VP of a chemical is 25 mm of Hg at 25 C, its VP at 35 C will be approximately 50 mm of Hg and at 45 C approximately 100 mm of Hg 15

16 Definition i i of Vapor Hazard Ratio Vapor Hazard Ratio (VHR) - The measure of a PURE material s ability to volatilize (expressed as vapor pressure) divided id d by the material s Occupational Exposure Level (OEL). VHR = vapor pressure OEL 16

17 Reference Popendorf, W., Vapor Pressure and Solvent Vapor Hazards. Am. IND. Hyg. Assoc. J. 45(10): (1984) Note: Reference also defines the term Vapor Hazard Index (VHI) as the log (VHR) Reference does not address mixtures 17

18 Use of Vapor Hazard Ratio If two chemicals have the same VHR, they will require the same level of control to assure exposures are not excessive. The VHR is the Rosetta Stone of IH 18

19 Vapor Hazard Ratio Scale Vapor Pressure (mm of Hg) Vapor Hazard / OEL (ppm) Ratio Scale < to < 1 2 1to< to < to < >

20 Chemicals in the same Vapor Hazard Ratio Category Scale - 1 Scale - 4 cyclohexanol ammonia biphenyl benzene Scale - 2 Scale - 5 acetone ethylene oxide aniline methyl mercaptan Scale - 3 Scale - 6 toluene - 2,4 - chlorine diisocyanate (TDI) phosgene n - hexane 20

21 Vapor Hazard Ratio - Controls Vapor Pressure Vapor Required Levels of Control (mm of Hg) / OEL (ppm) Hazard Ratio Scale < General Ventilation ~ 3to6air turnovers /hr 0.05 to < 1 2 Good general ventilation ~ 6 to 12 air turnovers/hr (GGV) 1 to < 25 3 GGV with capture at emission points 25 to < Capture at points of emission with containment wherever practical 500 to < Containment > Primary and Secondary Containment 21

22 Examples MEK has a VP = 86.7 mm of Hg and an OEL of 200 ppm, VHR = 0.42 requires good general ventilation ~ 6 to 12 air turnovers/hr (GGV) Methylene chloride has a VP = 430 mm of Hg and an OEL of 25 ppm, VHR = 17.2 requires GGV with capture at emission points Hexachlorocyclopentadiene(C56) has a VP= 0.06 mm of HG and an OEL of 0.01 ppm, VHR = 6 requires GGV with capture at emission points Benzene has a VP = 95.2 mm of Hg and an OEL of 0.5 ppm, VHR= Capture at points of emission with containment t wherever practical 22

23 Examples If a plant would like to purchase cyclohexane for a process at 25 C. Cyclohexane s VP = 96.0 and its OEL is 100 ppm. The process ventilation is at scale 3 (GGV with capture at emission i points). Will exposures be acceptable? The VHR = 0.96 which on the border between scale 2 and 3. Controls are adequate and the exposure band will likely be 2. 23

24 Examples Assume that the plant has a good exposure assessment illustrating that the cyclohexane exposure is 25 ppm or exposure band 2. If the plant would like to use MEK in the process, what would be MEK s exposure level and what would be its exposure band. Note that MEK s VHR=0.42, meaning that the controls would be more than adequate for MEK. The likely MEK exposure can be estimated by the following equation: MEK Exposure = [VHR MEK /VHR Cyclo ] x [Cyclo Exposure/OEL Cyclo ] x OEL MEK The cyclohexane exposure is currently 25% of its OEL and therefore the MEK exposure = [0.42/0.96] x 0.25 x 200 ppm = 22 ppm. MEK exposures would be at the low end of exposure band 2. 24

25 Particulates, Fumes and Mists Define a Potential Hazard Ratio analogous to the VHR. 25

26 Potential Hazard Ratio Scale for Solids and Mists Particle size Particle density Drop size Stratification by OEL 26

27 Potential Hazard Ratio Potential Hazard Ratio (PHR) Scale > 5 1 OEL Range (mg/m 3) 5to1 2 1 to t 0.1 to to

28 Potential Hazard Ratio - Controls OEL Range Potential Required Levels of Control (mg/m 3) Hazard Ratio Scale >5 1 General ventilation ~ 2 to 4 air turnovers/hr 5 to 1 2 Good General + fans ~ 4t to 6 air turnovers/hr 1 to Good General + fans ~ 6 to 8 air turnovers/hr 0.1 to Capture 0.01 to Containment Secondary containment 28

29 Rule of Thumb - Dustiness If particles are very small (high degree of dustiness), increase the PHR scale by 1. For example, if an chemical agent has an OEL of 0.5 mg/m 3 (PHR Scale 3) and is dusty, increase the expected controls to a PHR Scale 4. 29

30 Using Determinants of Exposure to Determine Exposure Band 30

31 Determinants of Exposure Environmental determinants: Type of control Efficiency of control Canopy hood vs. laboratory hood Frequency and duration of exposure Distance form source Size of container opening Surface area 31

32 Determinants of Exposure Agent Determinants: Agent surface area Vapor hazard index Composition Quantity of agent Absorption rate Application method 32

33 Identify Determinant Rating Scale Using Specified Definitions i i Exposure Control Frequency and Duration Vapor Hazard Ratio 33

34 Control Rating Scale Definitions i i Scale ESTIMATION OF OVERALL LEVEL OF CONTROL (Circle) 0 Closed system; no potential for release to work area 1 Primarily closed systems with effective engineering controls are in place to control exposure at potential contact points. 2 Open system; effective engineering controls in place to contain/remove airborne contaminants from work environment. 3 Combination open and closed system; a combination of engineering and administrative controls in place to control exposures 4 Open system; ineffective or no engineering controls in place 5 Open system; no program in place to minimize worker exposures; visual airborne contaminants, odors or sensory response indicate potential exposure exists 34

35 Frequency and Duration Scale Definitions i i Scale ESTIMATION OF OVERALL FREQUENCY (Circle) 1 Stressor is present less than 1 day/month; or less than 5 minutes/day 2 Stressor is present at least one day per month as follows: for 8 hour shifts, 5 minutes to 1 hour/day; for 12 hour shifts, 5 minutes to 1.5 hours/day 3 Stressor is typically present; for 8 hour shifts, 1 to 2 hours/day; for 12h hour shifts, to 3h hours/day 4 Stressor is typically present; for 8 hour shifts, 2 to 4 hours/day; for 12 hour shifts, 3 to 6 hours/day 5 for 8 hour shifts, 4 to 8 hours/day; for 12 hour shifts, 6 to 12 hours/day STEL Identify tasks that are conducted infrequently such as less than one day/month 35

36 Vapor Hazard Ratio (VHR) Scale Definitions i i Vapor Vapor Pressure ( mm of Hazard 25C) / OEL (ppm) Index Scale 1 < < < < >

37 Exposure Rating Estimation Exposure Rating Estimation (ERE) = (Control Rating) X (Frequency and Duration Rating) X (Vapor Hazard Index Rating) 37

38 Determine Exposure Rating or Band Exposure Rating Estimation (ERE) Exposure Rating (ER) < >

39 Exposure Rating Exposure Rating (Note: This rating system is not AIHA s) Statistical Interpretation 0 X x Exposure Limit (EL) x EL < X x EL x EL < X x EL x EL < X EL 4 X 0.95 > EL 39

40 Example Assume the following mixture: Chemical Weight % OEL (ppm) Molecular Weight (MW) Pure Vapor Pressure (VP) in torr at 25 C Tl Toluene (Tol) (Tl) Xylene (Xy) ethyl acetate (EA) Benzene (BZ) methylene chloride (MeCl) Carbon tetrachloride (CCl4)

41 Controlling Component Which component is controlling? That is, in this mixture which component has the highest potential to exceed its corresponding OEL. 41

42 Example Assume the following mixture: Chemical Weight % OEL (ppm) Molecular Weight (MW) Pure Vapor Pressure (VP) in torr at 25 C Tl Toluene (Tol) (Tl) Xylene (Xy) ethyl acetate (EA) Benzene (BZ) methylene chloride (MeCl) Carbon tetrachloride (CCl4)

43 Chemical WT % Calculations l A B C D E F G H I OEL MW VP Mole Mole % Corrected PHR PHR Norm. (ppm) Fraction VP (A/C) (E/ Total E) (F x D) (G/B) (H/ Largest H) x 100 Tol % Xy EA Bz MeCl CCl Total:

44 Why Do We Need Rules-of-Thumb? Consider the following cases. 44

45 Scenario A plant uses high purity toluene as a solvent in a process that t involves 35 workers. It is 4:30 pm on a Friday afternoon and you, the IH, and your family will be leaving on a week long vacation at the shore early the next morning. You must leave the plant no later than 5:00 pm and you have another 3 or 4 items on your task list that have to be complete before you leave. A process engineer comes into your office and asked you to sign off on using a lower grade of toluene that contains up to 1% benzene. Using this lower grade of toluene will save the plant $250,000/yr. 000/ The operations department t has already concluded that the lower grade toluene is acceptable and the safety and environmental departs have also already signed off on the change. 45

46 Scenario The engineer indicates that he realizes that the OSHA hazard communication and benzene standards will require some new labels and that the workers will have to be trained and that these requirements will be completed prior to receiving the new material in the plant. The plant will need to order a shipment no late than Saturday. 46

47 Scenario Currently, the toluene is monitored bimonthly and the data collected over the last year indicates the exposure is 9.5 ppm (95th percentile, GM=2.10, GSD=2.5, N=6). Pertinent information: Toluene s TLV = 20 ppm, BP=110.6 C, VP=28.44 mm of Hg, MW=92.1 Benzene s TLV = 0.5 ppm, BP=80.1 C, VP=95.18 mm of Hg, MW=78.1 Do you sign off on the change? 47

48 The vapor hazard ratio (VHR) of toluene is 28.4 mm of Hg / 20 ppm = The vapor pressure of benzene is suppressed because it is a small component of a mixture but the corrected vapor pressure can be calculated using Raoults law. Therefore the VHR for benzene = 1.12 mm of Hg / 0.5 ppm =

49 VHR bz / VHR tol = 2.24/1.12 = 1.58 Currently toluene exposures are 9.5 ppm/20 ppm = 47.5 % of the OEL Benzene exposures will = 1.58 X 47.55% of it corresponding OEL of 0.5 ppm Therefore benzene exposures can be expected to = 1.58 X X 0.5 = 0.38 ppm. 49

50 Although this value is below the benzene TLV it is above the action level which usually triggers medical surveillance requirements along with other requirements. It would be my recommendation as an IH to not approve the use of toluene with 1% benzene. A known human carcinogen would be introduced into the workplace where a significant number of employees would be affected, at a significant ifi level l of exposure and there would likely be a series of other costs that would have a negative impact on the projected savings. 50