AU5165898A - Device for measuring the concentration of airborne fibers - Google Patents

Description

DEVICE FOR MEASURING THE CONCENTRATION OF AIRBORNE FIBERS

Cross-reference To Related Application

This application is a continuation-in-part application of U.S. Patent Application Serial No. 08/743,554, entitled "Device For Measuring The Dimension Of A Airborne Fiber", filed on November 4, 1996 and U.S. Patent Application Serial No. 08/743,555, filed on November 4, 1996, which applications are assigned to the same assignee hereof, and are hereby incorporated by reference.

Field Of The Invention

This invention relates to methods and devices for estimating the concentration of airborne fibers, and particularly to devices which can decipher between respirable fibers and non-fibrous respirable fibers.

Background Of The Invention

At present, two primary methods for monitoring airborne fiber concentration exist. In the first method, airborne fibers are collected on a filter. This filter is analyzed by microscopy or chemical methods to determine the type of fibers present and to estimate airborne fiber concentration. This method suffers from the drawbacks of delayed availability of information, tediousness, inconvenience, high cost per sample, and lack of precision. Also, identification of fibers typically is performed by visual inspection, adding uncertainty to measurements for particular species of airborne fibers.

In the second method, real-time airborne fibers concentration is determined using optical techniques, in which light, attenuated by fibers passing by a light source, is analyzed. However, most of these devices do not discriminate between different species of airborne fibers and, in particular, may not provide an accurate measurement of potentially respirable fibers, particularly small glass fibers.

Because of the significant health problems posed by airborne asbestos fibers, current real-time airborne fiber monitors typically are aimed at selectively determining asbestos fiber concentration in an air sample having asbestos and other fibers. Because asbestos fibers exhibit paramagnetic properties, some existing devices preferentially align and oscillate asbestos fibers using, for example, a time-varying electric field quadrupole, a hybrid electric/magnetic field, or both. The induced oscillations tend to create a characteristic scattering of an impinging light, thus identifying the oscillating fiber as asbestos. Electrostatic techniques also may be used. Examples of such devices and methods for measuring airborne particulate concentration are found in U.S. Patent No. 3,692,412 to Chubb (1972), entitled "Apparatus for Analyzing Suspended Particles"; in U.S. Patent No. 4,940,327, to Lillienfeld (1990), entitled "Method and Apparatus for Real-Time Asbestos Monitoring"; and in U.S. Patent No. 5,319,575, also to Lillienfeld (1994), entitled "System and Method for Determining and Outputting Airborne Particle Concentration. " Also see MIE Fiber Monitor Model FM-7400 User's Manual by MIE, Inc., Billerica, MA.

However, because potentially harmful respirable fibers including, for example, glass fibers, often do not exhibit paramagnetism, such methods may not be appropriate. What is needed, then, is an airborne fiber concentration measuring device that can accurately determine the concentration of respirable fibers suspended in an air sample, in real time, without the need for electrostatic, magnetic or hybrid electromagnetic components.

Additionally, the Lillienfeld's device is more complicated, detects only a small percentage of fibers in a given sample, and if the concentration of fibers in the sample is low or not representative of the fiber concentration in the air flow, measurement errors can result. There therefore remains a need for a fiber concentration measuring device which takes a more significant sampling of the fiber population and which is accurate at low concentration readings.

Summary Of The Invention

This invention provides devices and methods for measuring the concentration of airborne fibers in a fiber-containing air sample. The preferred device includes flow means for providing laminar flow to at least a portion of the fibers in the air sample. These laminarly flowing fibers are then illuminated with a light source to produce scattered light. A portion of the scattered light is then sensed to produce an output from which a fiber concentration estimate can be measured. Additionally, separation devices can be used to preselect fibers having a particular size, so as to measure only respirable fibers, for example. This invention provides an inexpensive way of measuring respirable fibers in a work environment, such as a glass insulation or mat-making facility.

In a more detailed embodiment of this invention, a device is provided for analyzing air having respirable fibers, and non-respirable fibers or non-fibrous particulate matter, or both. This device includes separation means for selectively removing respirable fibers from non-respirable fibers to produce a filtered air sample containing aligned respirable fibers. These aligned fibers are then illuminated to produce scattered light, which is collected by a light sensor to produce an electrical output. The device further includes processing means for providing a concentration estimate for the respirable fibers from the output of the light sensor.

Brief Description Of The Drawings

The accompanying drawings, referenced to herein and constituting a part hereof, illustrate preferred embodiments of the device of the present invention and, together with the description, serve to explain the principles of the invention.

Figure 1 is an illustration of an airborne fiber concentration measuring device in accordance with the present invention.

Figure 2 is an illustration of one presently preferred embodiment of a sensor in accordance with the present invention.

Figure 3 is an illustration of another presently preferred embodiment of a sensor in accordance with the present invention.

Detailed Description Of The Invention

Figure 1 illustrates one embodiment of the airborne fiber concentration measuring device 100 according to the principles of the invention herein. Device 100 can include a sensor 1 for detecting fibers and separation means, for example, virtual impactor 2, for separating respirable from non-respirable fibers or non-fibrous particulate matter. As used herein, "respirable fibers" means fibers which are less than about 3μM in diameter, and preferably those with an aspect ratio of at least about 5:1 (length: diameter). Additionally, the term "light" refers to both visible and invisible electromatic waves, including x-ray and infrared.

A skilled artisan would recognize that virtual impactor 2 can use well-known techniques to separate the respirable particles from non-respirable particles, and therefore, the skilled artisan could employ other separating means for isolating respirable fibers from non- respirable fibers. One exemplary virtual impactor 2 that has been found suitable is shown in FIG. 1. This device takes in fiber-containing ambient air and draws off smaller respirable fibers 20 laterally at a venturi's mouth. Larger fibers 41 , greater than about 3μm, are drawn into the center tube of the virtual impactor 2.

In general, the air entering the device can have respirable fibers, non- respirable fibers, and other particulate matter mixed therein. Sensor 1 preferably senses aligned respirable fibers in the air but is substantially insensitive the other non-fibrous particulate matter. In operation, respirable fibers 20 that may be present in the air are drawn from virtual impactor 2 through hose 3 which connects virtual impactor 2 to sensor 1. Air is drawn through the system by a small vacuum pump 22 to outlet 4 of lower flow tube 6. The air flow rate, and lengths and diameter of the upper and lower flow tubes 5,6, are preferred to be such as to produce a laminar flow of air through tubes 5,6. This laminar airflow tends to cause the fibers 20 in the air within tubes 5,6 to become substantially aligned with the airflow and, hence, with the longitudinal axis 30 of flow tubes 5,6. Flow tubes 5,6 preferably are separated by a small gap 7 within sensor 1. Alternatively, a single tube having a pair of slots through its side wall perpendicular to its axis could work as well. This gap 7 is preferably positioned symmetrically about axis 8 of sensor 1. Flow tubes 5,6 and gap 7 constitute the "flow channel" for this embodiment of the invention.

Within sensor 1 is a light source 9 which can be a coherent light source such as, for example, a diode laser. Light source 9 can produce a beam 12, preferably with a preselected cross-section along the beam path. It is preferred that light source 9 produce a collimated beam of light, ideally with an elliptical cross-section directed at light sensor 14. Light sensor 14 is preferred to be a photodetector. Beam 12 can be aimed along axis 8 of sensor 1 with the major axis of the ellipse of light preferably being substantially parallel to gap 8 between flow tubes 5,6. The width of beam 12 need not be as wide as the diameter of flow tubes 5,6.

A suitable light source for this embodiment can be, for example, a model LPM 03(670-5) laser diode from Power Technology, Inc., Little Rock, Arkansas. Similarly, a suitable photodetector is, for example, Devar Model 509-1, Bridgeport, Connecticut. A skilled artisan could employ other suitable light sources and light sensors to provide and detect light signals indicative of the presence of respirable fiber.

Figure 2 presents a cross-sectional view of a preferred sensor 1 , which is positioned generally perpendicular to the airflow. After passing through gap 7, beam 12 enters an optical lens assembly 10. Lens assembly 10 can be a pair of condensing lenses, for example. This combination of lenses tends to have a short focal length, permitting a portion 23 of beam 12 to be directed to the back surface 24 of the second lens 25. Beam block 11 can be used to substantially block the collimated light 23 from being sensed by photodetector 14. It is preferred that the beam block 11 be umbrageously situated relative to photodetector 14 so that beam block 11 can shield photodetector 14 from light not indicative of the presence of a sensed fiber.

As fibers 20 pass though the beam 12 between the flow tubes 5,6, some of the fibers 20 will scatter the light, as shown in FIG. 2. When a cylinder, such as a glass fiber, is illuminated at a normal incidence by light, it typically scatters the light in a preselected orientation in the flow channel, i.e. in a plane that is normal to the cylinder. Because fibers 20 have been aligned by the laminar airflow, these fibers 20 are generally oriented perpendicularly to the direction of beam 12. Therefore, beam 12 can be scattered in a plane that is generally parallel to planes formed by the ends of flow tubes 5,6, thus permitting scattered light 26 to pass through gap 7 between flow tubes 5,6.

For the laminar flow of this invention it is generally recognized that two conditions must be met. These are that the Reynolds number should be less than about 2000 and there must be sufficient distance for the flow to become laminar. In the case of the claimed device, a flow of about 4 liters/min. and a fiber diameter of .44 in. (1.1 cm) produces a Reynolds number of about 500, which is well into the laminar flow regime. The length of the flow tube before the fibers reach the laser beam is about 5-50 in. (12.7-127 cm), preferably about 10 in. (25.4 cm) which is more than 22 times the fiber diameter. Since laminar flow should develop within 10 diameters from the entrance of the tube the flow in the device should have ample time to assume a laminar condition.

A visual confirmation of the alignment of fibers during the transition between turbulent flow and laminar flow can be made. It can be seen that: in the case of glass fibers in a turbulent flow, the diffracted laser beam is dispersed into separated spots of light in random directions; while in the case of glass fibers in a laminar flow, the diffracted laser beam is concentrated in approximately one direction (area), thus showing that the fibers are aligned in a direction substantially parallel to the fiow.

Light that is scattered in a forward direction 13 can be collected by lens assembly 10 and focused on photodetector 14. Because this light typically is not collimated when it enters the lens assembly 10, it can be focused to a point some distance beyond lens assembly 10, thereby passing around beam block 11. Thus, while both the beam 12 and scattered light 26 enter lens assembly 10, beam 12 typically is blocked from impinging on photodetector 14 while scattered light 26 is, for the most part, focused onto the photodetector 14. Overall, only a small fraction of scattered light 26 is blocked by beam block 11.

It is preferred that photodetector 14 have a sensing region with a finite width which is wide enough to receive the scattered light 26. Within this width, it will respond to light scattered by fibers 20 that are some distance to either side of, as well as in front and in back of, axis 30 of flow tubes 5,6. Therefore, fibers 20 are not required to pass through beam 12 single-file or closely aligned with axis 30. When beam 12 is scattered by fiber 20, it is focussed though lens assembly 10 to impinge upon photodetector 14, thus generating a brief electrical pulse therefrom. In general, the amplitude of this pulse is preferred to be proportional to the amount of light scattered by the fiber. The resultant pulse can be sent to an appropriate electronic measurement circuit 31 where the pulse is recorded. Using other quantitative information, such as, the flow rate of the air through sensor 1, and determining the rate at which the pulses are received, the concentration of respirable fibers in the air can be determined.

It is preferred that sensor 1 be substantially insensitive to non-fibrous particulate matter. Presently preferred embodiment of the current invention accomplish this selectivity by analyzing, for example, the optical differences between the typically cylindrical respirable fibers, and particulate matter having other shapes. That is, if a spherical or irregularly-shaped dust particle is drawn into sensor 1 , the particulate matter will also scatter light from beam 12. However, such a particle tends to scatter light into a spherical volume. Much of this scattered light will impinge on, and be absorbed by the walls of flow tubes 5,6.

In general, only a small fraction of the light scattered by these particles tends to pass through the gap 7 between flow tubes 5,6. This small amount of scattered light tends to produce only a weak signal in photodetector 14. Circuit 31 , receiving pulses from the photodetector 14, can be designed to ignore low amplitude pulses resulting from particulate matter. Therefore, device 100 can be made to respond only to respirable fibers while ignoring other non-fibrous particulate matter that may be present. Unlike prior art devices, the invention herein does not require the use of electrostatic or electromagnetic components to induce movement in the matter suspended in the air in order to determine whether or not the matter is a respirable fiber.

Indeed, the ability of device 100 to discriminate between respirable fibers and other particles could optionally use the following principles. First, non-respirable fibers are eliminated from the airflow by separation means, i.e. virtual impactor 2, before the air enters sensor 1. Second, the remaining fibers tend to be aligned with flow tube axis 30 by the laminar flow of air through tubes 5,6. Third, beam 12 generally is oriented to be normal to the axis of tubes 5,6. Fourth, light scattered by fibers 20 tends to be scattered in a plane which passes between the ends of flow tubes 5,6, and a portion of the scattered light is focused onto photodetector 14. Fifth, light scattered by other particles tends to be scattered more omni-directionally than is the case with cylinders. Most of this light is absorbed by the walls of flow tubes 5,6 and only a small amount of light remains to be focused on photodetector 14. Sixth, by discriminating between the amplitude of signals received from photodetector 14, device 100 can discriminate between fibers and other particles.

In Figures 1 and 2, lens assembly 10 and photodetector 14 are shown as being substantially in-line with, or in opposition to, beam 12. In view of the teachings of this invention, a skilled artisan would recognize that lens assembly 10 and photodetector 14 may be placed anywhere around axis 30 of flow tubes 5,6, as long as they are still in the plane of light scattered from fibers 20. Although the amount of light collected by lens assembly 10 can depend upon the location of lens assembly 10, sensor 1 can discriminate between respirable fibers and other particles even with these alternative configurations.

In Figure 3, for example, the components of device 100 are substantially the same as those in Figures 1 and 2, with the exception that lens assembly 10 and photodetector 14 have been rotated in orientation by 90 degrees. Also in Figure 3, beam block 11 seen in Figures 1 and 2, may be eliminated because beam path 12 no longer is in-line with, or in opposition to, photodetector 14.

All publications mentioned in this specification are indicative of the level of skill of the skilled in the art to which this invention pertains. All publications are herein incorporated by reference to the same extent as if each individual publication was specifically but individually indicated to be incorporated by reference.

While specific embodiments of practicing the invention have been described in detail, it will be appreciated by those skilled in that art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Indeed, a skilled artisan would recognize that, although the invention has been described in terms of determining the concentration of airborne respirable fibers, the apparatus and method illustrated in detail herein also can be used to detect, characterize, and visualize other types of particles having specific optical properties. Accordingly, the particular arrangements of the methods and apparatus disclosed are meant to be illustrative only and not limiting to the scope of the invention, which is to be given the full breadth of the following claims, and any and all embodiments thereof.

Claims (35)

What is claimed is:

1. A device 100 for measuring the concentration of respirable airborne fibers in a fiber-containing air sample, said device 100 characterized by: a. flow means for providing laminar flow to at least a portion of the fibers in said air sample. b. a flow channel 5, 6 for receiving a plurality of laminarly flowing fibers

20; c. a light source 9 for generating a light beam 12 directed to said plurality of laminarly flowing fibers 20 to produce scattered light; and d. a light sensor 14 for sensing a portion of said scattered light and for generating an output from which a respirable fiber concentration estimate in said air sample can be measured.

2. The device of claim 1 , wherein the sampled air also includes non-respirable fibers and non-fibrous particulate matter therein.

3. The device of claim 1 , wherein said device has a slotted opening for channeling scattered light to said light sensor.

4. The device of claim 1, wherein said light source includes collimation means for providing a light beam having a preselected cross-section along a beam path.

5. The device of claim 4, wherein said flow channel has a longitudinal axis which is substantially normal to the beam path.

6. The device of claim 3, wherein said slotted opening is disposed to screen out scattered light which does not have a preselected orientation to the light sensor.

7. The device of claim 6, further including an optical lens for receiving a portion of the scattered light having a preselected orientation and for directing at least a portion of this scattered light to the light sensor.

8. The device of claim 7, further including a beam block located in a path of said light beam, said beam block being umbrageously situated relative to the light sensor, the beam block substantially blocking the light beam from the light sensor.

9. The device of claim 1 , wherein said light sensor includes means for generating a first light pulse having a first amplitude which is representative of a respirable fiber and a second pulse having second amplitude which is representative of a particle which is not a respirable fiber, said device capable of discriminating between the first amplitude for said respirable fiber and the second amplitude for said particle, and providing a signal which is representative of a concentration of respirable fibers.

10. The device of claim 9, wherein said particle is a non-respirable fiber.

11. A method of measuring the concentration of airborne fibers in a fiber- containing air sample containing respirable and non-respirable fibers, characterized by: a. providing laminar flow to at least a portion of the fibers in said air sample, said portion containing respirable and non-respirable fibers; b. directing a light beam at said laminarly flowing respirable and non- respirable fibers to produce a scattered light; and c. sensing a portion of said scattered light and generating an output from which a respirable fiber concentration estimate can be produced.

12. The method of claim 11 wherein said sensing step includes providing first and second pulse signals corresponding to the detection of light from a respirable and a non- respirable fiber respectively, said first and second pulse signals having first and second amplitudes, said method further including discriminating between said first and second amplitudes to provide a signal representative of a concentration of respirable fibers in said air sample.

AMENDED CLAIMS

[received by the International Bureau on 26 March 1998 (26.03.98) ; original claims 1 and 11 amended; new claims 13-35 added; remaining claims unchanged (9 pages)]

What is claimed is:

1. A device 100 for measuring the concentration of respirable airborne fibers in a fiber-containing air sample, said device 100 characterized by: a. flow means for providing laminar flow to at least a portion of the fibers in said air sample; b. a flow channel 5, 6 for receiving a plurality of laminarly flowing fibers 20, said laminarly flowing fibers being substantially aligned with an airflow in said flow channel; c. a light source 9 for generating a light beam 12 directed to said plurality of laminarly flowing fibers 20 to produce scattered light; and d. a light sensor 14 for sensing a portion of said scattered light and for generating an output from which a respirable fiber concentration estimate in said air sample can be measured.

2. The device of claim 1, wherein the sampled air also includes non-respirable fibers and non-fibrous particulate matter therein.

3. The device of claim 1 , wherein said device has a slotted opening for channeling scattered light to said light sensor.

4. The device of claim 1. wherein said light source includes collimation means for providing a light beam having a preselected cross-section along a beam path.

5. The device of claim 4, wherein said flow channel has a longitudinal axis which is substantially normal to the beam path.

6. The device of claim 3, wherein said slotted opening is disposed to screen out scattered light which does not have a preselected orientation to the light sensor.

7. The device of claim 6, further including an optical lens for receiving a portion of the scattered light having a preselected orientation and for directing at least a portion of this scattered light to the light sensor.

8. The device of claim 7, further including a beam block located in a path of said light beam, said beam block being umbrageously situated relative to the light sensor, the beam block substantially blocking the light beam from the light sensor.

9. The device of claim 1, wherein said light sensor includes means for generating a first light pulse having a first amplitude which is representative of a respirable fiber and a second pulse having second amplitude which is representative of a particle which is not a respirable fiber, said device capable of discriminating between the first amplitude for said respirable fiber and the second amplitude for said particle, and providing a signal which is representative ofa concentration of respirable fibers.

10. The device of claim 9. wherein said particle is a non-respirable fiber.

11. A method of measuring the concentration of airborne fibers in a fiber-containing air sample containing respirable and non-respirable fibers, characterized by: a. providing laminar flow to at least a portion of the fibers in said air sample, said portion containing respirable and non-respirable fibers; b. directing a light beam at said laminarly flowing respirable and non- respirable fibers, said laminarly flowing fibers being substantially aligned with an airflow, to produce a scattered light; and c. sensing a portion of said scattered light and generating an output from which a respirable fiber concentration estimate can be produced.

12. The method of claim 11 wherein said sensing step includes providing first and second pulse signals corresponding to the detection of light from a respirable and a non- respirable fiber respectively, said first and second pulse signals having first and second amplitudes, said method further including discriminating between said first and second amplitudes to provide a signal representative ofa concentration of respirable fibers in said air sample.

13. A device 100 for measuring the concentration of respirable airborne fibers in a fiber-containing air sample, said device 100 characterized by: a. flow means for providing laminar flow to at least a portion of the fibers in said air sample. b. a flow channel 5, 6 for receiving a plurality of laminarly flowing non- oscillating fibers 20; c. a light source 9 for generating a light beam 12 directed to said plurality of laminarly flowing fibers 20 to produce scattered light; and d. a light sensor 14 for sensing a portion of said scattered light and for generating an output from which a respirable fiber concentration estimate in said air sample can be measured.

14. The device of claim 13, wherein the sampled air also includes non-respirable fibers and non-fibrous particulate matter therein.

15. The device of claim 13, wherein said device has a slotted opening for channeling scattered light to said light sensor.

16. The device of claim 13, wherein said light source includes collimation means for providing a light beam having a preselected cross-section along a beam path.

17. The device of claim 16, wherein said flow channel has a longitudinal axis which is substantially normal to the beam path.

18. The device of claim 15, wherein said slotted opening is disposed to screen out scattered light which does not have a preselected orientation to the light sensor.

19. The device of claim 18, further including an optical lens for receiving a portion of the scattered light having a preselected orientation and for directing at least a portion of this scattered light to the light sensor.

20. The device of claim 19, further including a beam block located in a path of said light beam, said beam block being umbrageously situated relative to the light sensor, the beam block substantially blocking the light beam from the light sensor.

21. The device of claim 13 , wherein said light sensor includes means for generating a first light pulse having a first amplitude which is representative ofa respirable fiber and a second pulse having second amplitude which is representative of a particle which is not a respirable fiber, said device capable of discriminating between the first amplitude for said respirable fiber and the second amplitude for said particle, and providing a signal which is representative ofa concentration of respirable fibers.

22. The device of claim 21 , wherein said particle is a non-respirable fiber.

23. A method of measuring the concentration of airborne fibers in a fiber-containing air sample containing respirable and non-respirable fibers, characterized by: a. providing laminar flow, without oscillation, to at least a portion of the fibers in said air sample, said portion containing respirable and non-respirable fibers; b. directing a light beam at said laminarly flowing non-oscillating respirable and non-respirable fibers to produce a scattered light; and c. sensing a portion of said scattered light and generating an output from which a respirable fiber concentration estimate can be produced.

24. The method of claim 23 wherein said sensing step includes providing first and second pulse signals corresponding to the detection of light from a respirable and a non- respirable fiber respectively, said first and second pulse signals having first and second amplitudes, said method further including discriminating between said first and second amplitudes to provide a signal representative of a concentration of respirable fibers in said air sample.

25. A device 100 for measuring the concentration of respirable airborne fibers in a fiber-containing air sample, said device 100 characterized by: a. flow means for providing laminar flow to at least a portion of the fibers in said air sample. b. a flow channel 5, 6 for receiving a plurality of laminarly flowing fibers 20, said flow channel having a longitudinal axis; c. a light source 9 for generating a light beam 12 directed to said plurality of laminarly flowing fibers 20 to produce scattered light, said light beam having a beam path substantially normal to said longitudinal axis of said flow channel; and d. a light sensor 14 for sensing a portion of said scattered light and for generating an output from which a respirable fiber concentration estimate in said air sample can be measured.

26. The device of claim 25, wherein the sampled air also includes non-respirable fibers and non-fibrous particulate matter therein.

27. The device of claim 25, wherein said device has a slotted opening for channeling scattered light to said light sensor.

28. The device of claim 25, wherein said light source includes collimation means for providing a light beam having a preselected cross-section along a beam path.

29. The device of claim 27, wherein said slotted opening is disposed to screen out scattered light which does not have a preselected orientation to the light sensor.

30. The device of claim 25, further including an optical lens for receiving a portion of the scattered light having a preselected orientation and for directing at least a portion of this scattered light to the light sensor.

31. The device of claim 30, further including a beam block located in a path of said light beam, said beam block being umbrageously situated relative to the light sensor, the beam block substantially blocking the light beam from the light sensor.

32. The device of claim 25, wherein said light sensor includes means for generating a first light pulse having a first amplitude which is representative of a respirable fiber and a second pulse having second amplitude which is representative ofa particle which is not a respirable fiber, said device capable of discriminating between the first amplitude for said respirable fiber and the second amplitude for said particle, and providing a signal which is representative of a concentration of respirable fibers.

33. The device of claim 32, wherein said particle is a non-respirable fiber.

34. A method of measuring the concentration of airborne fibers in a fiber-containing air sample containing respirable and non-respirable fibers, characterized by: a. providing laminar flow in a flow direction to at least a portion of the fibers in said air sample, said portion containing respirable and non-respirable fibers; b. directing a light beam at said laminarly flowing respirable and non- respirable fibers to produce a scattered light, said light beam having a beam path substantially normal to said flow direction; and c. sensing a portion of said scattered light and generating an output from which a respirable fiber concentration estimate can be produced.

35. The method of claim 34 wherein said sensing step includes providing first and second pulse signals corresponding to the detection of light from a respirable and a non- respirable fiber respectively, said first and second pulse signals having first and second amplitudes, said method further including discriminating between said first and second amplitudes to provide a signal representative of a concentration of respirable fibers in said air sample.