Exposure Controls for physical Agents

Transcription

1 Exposure Controls for physical Agents

2 Nature of physical agents Physical agents usually involve the transfer of energy from a source to a human receptor The energy can be in various forms: Mechanical: noise, vibration, ultrasound Electrical: electric shock Electromagnetic NIR: UV &, visible light, Infrared/heat, RF/microwaves Ionizing: X-rays, γ radiation, Energetic particles: α, β, neutrons, protons

3 Monitoring physical agents Many health effects due to physical agents can be related to energy deposition in tissue Energy deposition often related to damage The wavelength or frequency often play a significant role in the energy deposition Damage and health effects often are non-linear in relation to energy deposition Exposure often is measured in units of energy or power received at the receptor Examples: db, light intensity, power density Exposure limits applied in the same units

4 Physical Agents: Control strategies Control strategies for physical agents generally follow those for other chemicals Control at source, pathway, or receptor For physical agents we want to limit the transfer of energy (or power) to the receptor Limit the power emitted by the source Lower the power transmitted over the pathway Reduce the power absorbed at the source

5 Energy transfer 4 basic interactions: transmission, absorption, reflection, scattering Absorption causes heating or other effects due to energy deposition Energy transfer depends on wavelength, polarization and geometry

6 Example: Noise Control AT THE SOURCE: Acoustical design Decouple energy from vibrating system Change coupling between energy and acoustical radiating system Change structure so less sound is radiated Substitute less noisy equip. Change processing methods CHANGES IN PATH: Increase source-receiver distance Acoustical treatment of ceiling, walls & floor (reverberation) Enclose noise source AT RECEIVER BY Personal Protection Enclosure for worker Job rotation schedule Work hours

7 Control points at: Plant design At the source Along the path NOISE CONTROL AND REDUCTION At the Receiver

8 CONTROL AT THE SOURCE SUBSTITUTE for quieter equipment, processes or materials REDUCE DRIVING FORCE by reducing motion, maintaining dynamic balance, increasing duration of work cycle, and providing vibration isolation REDUCE RESPONSE of vibrating surfaces by dampening and decreasing radiating area REDUCE VELOCITY flow of fluids

9 NOISE REDUCTION ALONG SOUND PATH Move position of the person or noise source Change acoustics of the environment Room absorption Provide barriers between noise source and receiver Enclosures Mufflers Sound shields

10 SOUND ATTENUATION WITH DISTANCE Outdoors, no reflecting surfaces: Where: SPL 1 is the. level at distance D 1 SPL 2 is the level at distance D 2 General rule: 6dB reduction for each distance doubled

11 Enclosure Design Guidelines Air tight enclosure works best Acoustical absorption added to the interior of the enclosure is essential for effectiveness Enclosures are improved by using heavier walls A single leak can destroy the enclosure effectiveness Use of porous materials are of little value Acoustical absorbing materials on the outside of an enclosure does not improve the effectiveness Vibrations transmitted from the source to the enclosure can destroy enclosure effectiveness

12 Absorption Coefficient (α) Where E i = the incident energy E r = the reflected energy E a = the absorbed energy ABSORBANCE (A) = αs Note: Absorbance is measured in sabins where 1 sabin = 1 sq ft of perfect absorbance

13 Sound absorption coefficients of common acoustic materials

14 Sound absorption coefficients of common construction materials

15 Noise Reduction by Absorption NR = 10 log A 2 /A 1 Where: NR is the noise reduction (db) A 1 is the initial absorption sabins, A 2 is the final absorption (sabins)

16 Calculating room absorption A room = Total number of absorption units in the room (Sabins) A room = A ceiling + A wall + A machinery + A floor A room = S ceiling α ceiling + S wall α wall + S machinery α machinery + S floor α floor 1. Calculate areas of ceilings walls, floor, other large surface area machinery separately 2. Determine appropriate absorption coefficient for each type of surface 3. Multiply the area and absorption coefficient for each type of surface and sum the total of these products to determine A room 4. Calculate A room before treatment using absorption coefficients of the old materials 5. Calculate A room after treatment using absorption coefficients of the new materials 6. Calculate NR = 10 1og(A room, after treatment ) / (A room, after treatment ) 7. Calculations must be completed for all octave band center frequencies

17 Absorption in relation to R

18 Room Constant (R) Note: R is the measure for how live a room or an enclosure is.

19 Transmission Coefficient (τ) Where E i is the incident energy E r is the reflected energy and E t is the transmitted energy Transmission Loss (TL) Energy lost (TL) = E i -E t TL = Incident L w Transmitted L w

20 Transmission loss of common materials

21 Transmission loss from enclosures Transmission Loss (TL) The amount of energy lost (in db) as sound travels through an enclosure Determining required transmission loss 1) Determine the amount of attenuation needed 2) Allow for reverberant buildup or provide enclosure treatment with an absorption coefficient of >0.7 3) For each frequency, subtract the TL from the SPL 4) Add the resultant sound pressure levels to determine total transmission loss

22 Hearing Protectors Sound pathways through protectors 1. Air leaks 2. Hearing protector vibration 3. Material Transmission 4. Bone/tissue conduction Muffs Inserts

23 Comparing Hearing Protectors Insert Type Protectors Advantages small, handy, low cost no interference (i.e. hair, glasses) comfort in hot environments no restrictive head movement Disadvantages fitting effect amount of protection available dirty environment cannot be seen Muff Type Advantages >attenuation, < variability one size fits most easy to monitor more readily accepted can be worn despite minor ear infections not easy to lose Disadvantages uncomfortable in hot environments not easily carried not convenient (i.e.. glasses) may restrict head movement higher cost exposed surface area limits attenuation above 1 khz

24 Noise Reduction Rating (NRR) Hearing Protector Attenuation Method #1 1) Obtain employee s C-weighted TWA 2) Subtract NRR to determine estimated A-weighted TWA under the ear protector Method #2 1) Obtain employee s A-weighted TWA 2) Subtract 7 db from the NRR 3) Subtract remainder from the A-weighted TWA to determine A-weighted TWA under the ear protector

25 END Noise Part

26 Lasers This image is copyrighted.

27 Laser Hazard Classification

28

29 Types of laser hazards 1. Eye : Acute exposure of the eye to lasers of certain wavelengths and power can cause corneal or retinal burns (or both). Chronic exposure to excessive levels may cause corneal or lenticular opacities (cataracts) or retinal injury. 2. Skin : Acute exposure to high levels of optical radiation may cause skin burns; while carcinogenesis may occur for ultraviolet wavelengths ( nm). 3. Chemical : Some lasers require hazardous or toxic substances to operate (i.e., chemical dye, excimer lasers). 4. Electric shock : Most lasers produce high voltages that can be lethal. 5. Fire hazards : The solvents used in dye lasers are flammable. High voltage pulse or flash lamps may cause ignition. Flammable materials may be ignited by direct beams or specular reflections from high power continuous wave (CW) infrared lasers.

30 Reflected beam caused vision loss Description: Professor from China removed eyewear to "see better" while doing an experiment with a crystal. Exposure produced retinal burn and permanent vision loss. He described seeing a white flash, central purple spot surrounded by yellow halo. No pain reported.

31 Retinal burn from rear laser mirror Description: Student WITH EYEWEAR ON (and witness to verify) received exposure from the rear mirror of a "Continuium" YAG laser. The student was wearing Glendale Broadband (OD 4.0) eyewear; ANSI standard requires OD=6.0. Retinal burn resulted with permanent damage. Type:Nd:YAG Divergence:- Wavelength:532 nm Energy/Power:0.18/0.40 Class:- Pulse Rate:5 KHz Exposure Time:7 ns

32 Example of eye injury Experience has demonstrated that most laser injuries go unreported for hours by the injured person. This is a critical time for treatment of the injury.

33 Retinal Burn A range of injuries induced with a Nd:YAG laser on a monkey retina. The white spots in the centre are thermal burns, i.e. coagulation of retinal layers. With larger energies, holes in the retina are produced which result either in bleeding into the vitreous (the gel-like substance which fills the centre of the eye ball), or the bleeding is contained in the layers of the retina, which results in functional loss in the affected area. Photograph courtesy of J. Zuclich, TASC Litton, TX, USA.

34 Eye Damage: Focusing Your Eyes Are Designed to Focus With safety rule Cornea Damage BAD Retina Damage WORSE

35 Controls Enclosure form of interlocked rooms and protective housings Beams routed over or below walkways using mirrors and/or elevated enclosures or tunnels remote interlock connection triggers a shutdown emergency panic button shutoffs fasteners with special operating tools room status lights loud speakers/buzzers for visitors

36

37 This image is copyrighted.

38

39 Exposure Limits Retinal Injury Thresholds - I - At seconds the threshold for a retinal injury is ~ 10-7 J/cm 2 (i.e W/ cm 2 ). - Because of the x 10 5 enhancement in the eye this value is elevated to 10-2 J/cm 2 (i.e W/cm 2 ) on the retina. - These exposure levels are further enhanced by self-focusing.

40

41

42