Tracking Air Movement.

Question: How many times, while inspecting a food service establishment, have you been unsure about the operation of a canopy hood, oven exhaust, toilet ventilation, or general exhaust system? Tossing a small piece of facial tissue paper into the air near a hood opening to see if the ventilation system is on does not exactly demonstrate a high degree of professionalism. Even worse is firing up a cigarette and watching where the smoke goes. If you need an answer, use the proper detection system.

We have found two methods that help us visualize air movement. Each is user-friendly, relatively inexpensive, and readily available through the usual trade catalogs. Between them, these two methods provide us with an ample latitude of options for safe air-movement visualization in a manner appropriate to the facility or situation. They are as follows:

* Smoke tubes are similar in appearance to sorbent sample tubes, but they generate a persistent chemical smoke when air is pushed through them with a small aspirator bulb.

* Inert powder is dispersed in puffs, either through an aspirator bulb-powered "gun" or through a plastic squeeze bottle with a small opening.

Smoke Tubes

Two types of smoke tube are available. Drager system smoke tubes (with their marvelous original name of Stromungs-Prufrorchen) have a brownish-orange filling impregnated with fuming sulfuric acid. When humid air is pumped into a tube by means of a rubber bulb, sulfuric-acid aerosol emerges in the form of white smoke. Smoke tubes of the second type, manufactured by companies such as SKC, Inc., and Mine Safety Appliances Company (MSA), contain stannic chloride sorbed onto pumice. The chemical reacts with moisture in the air, yielding tin oxides, smoke, and hydrochloric acid. Both kinds of smoke tube produce a graphic, persistent smoke, which is irritating to the eyes and mucous membranes, We find the latter type, containing the stannic chloride, a bit more odious. In fact, a material safety data sheet (MSDS) comes with these tubes as a package insert. For this reason, we do not use either type of smoke tube in occupied areas without adequate exhaust ventilation, particularly when assessing indoor air quality pr oblems. In no case do we use this technology in patient-, student-, or resident-occupied hospitals, schools, nursing homes, or long-term care facilities. Smoke tubes are, however, ideal for evaluating slight air currents in workrooms and for defining approximate ventilation capture areas of hoods, including the canopy hoods found in most kitchens. They also are useful for monitoring ventilation and air-conditioning systems, particularly in laboratories and industries that rely on air-pressure gradients rather than clean-room technology for contamination control. At research facilities, the smoke tubes are absolutely essential to ensuring tight seals on devices or rooms that are subjected to microbiological decontamination with formaldehyde gas, such as Class II biological safety cabinets or BSL-3 laboratories and vivaria.

Smoke tubes are easy to use. First, position yourself upwind. Then, activate the tube by carefully breaking off both tips with the metal tube breaker provided in the kit. Next, insert the tube tightly into the rubber aspirator bulb, making sure the arrow points in the direction of air flow. Hold the assembly at the inlet end of the air stream and slowly squeeze the bulb to produce smoke. To estimate the approximate velocity of the air stream, note the time it takes the smoke to travel a measured distance. Each manufacturer supplies a different kind of aspirator bulb. Experience has demonstrated that an aspirator with a one-way valve has the longest useful life and prevents refluxing acid vapors. The kit sold by MSA has the best aspirator bulb.

Smoke tubes are ambient-condition dependent and require humidity of three to 50 milligrams per liter (mg/L), which corresponds to 100 percent relative humidity at 40[degrees]C. In other words, the amount of smoke produced is dependent on the ambient humidity of the area being evaluated; higher humidity produces more smoke. Smoke tubes therefore are ideal for use in most food service areas but of limited value in dry climates and in buildings without humidification during the colder months.

For storage or intermittent use over a longer period, remove the tube from the aspirator and close the ends with the plastic tube caps provided in the kit. To protect the contents, the caps must fit tightly on the ends of the tube, allowing no air to enter. Do not tuck a used smoke tube in your pocket, even if you intend to reuse it, and do not touch the ends of the tube after removing the protective caps; liquid acid may have accumulated in the tube and could leak out. Also, because the tube contents are corrosive and the glass ends are sharp, take extra care in their disposal and always follow the manufacturer's recommendations.

Inert Powder

The safest way to visualize air currents and track their movement is with an inert powder, which is neither a corrosive nor an irritant. While powder is not as persistent or as visible in the airstream as smoke, it has the distinct advantage of being relatively safe. The commonly used powders contain finely divided amorphous silicon dioxide that is biologically benign (it is the crystalline form of silicon dioxide that is associated with occupational illness), as well as zinc stearate, an ingredient found in cosmetics, ointments, and dusting powders. Although both compounds are inherently harmless except for some skin-drying effect, prolonged exposure without a mask should he limited. In any case, we found it prudent to take any MSDS or other safety information with us on our inspections. Invariably, someone always wants to know if the material could be harmful.

From an applications standpoint, powders are not humidity dependent. Also, we recommend using this technology in occupied areas. Air-tracking powder is designed for use in locating drafts, determining the velocity of slow-moving currents in a room, and obtaining a general picture of air motion. Although inert powders can be used to check air currents for ventilation capture in chemical-fume hoods, we do not recommend use of this method in biological safety cabinets because the powder might clog the high efficiency particulate air (HEPA) filters.

The delivery systems for inert powder are of two types: One is simple and inexpensive, and the other is rather elegant and a bit more costly. In this instance, we can state, emphatically, that you get what you pay for. The most common and less expensive delivery system is a handheld plastic squeeze container with a small opening. These systems contain amorphous silicon dioxide powder and are marketed under various names such as [Flowchecker.sup.TM] [*] The directions instruct users to aerate the powder by shaking the container and then to squeeze the center of the container with a quick, firm motion. We have never mastered the technique of squeezing without getting powder on everything, except when air movement is already quite discernible and the container is held close to the source.

On the other end of the delivery spectrum is the Tel [Tru.sup.TM] Borozin A.C. Gun, manufactured by E. Vernon Hill, Inc. [**] This system retails for about $130. The gun itself is polished stainless steel and comes complete with a supply of "Borozin" powder and a hard-side carrying case. The powder contains 100 percent air-test-grade zinc stearate. The zinc stearate seems to be lighter and more finely divided than silicon dioxide and therefore more persistent in an airstream. The gun dispenses the powder with a simple rubber bulb attached in the same position as a grip on a handgun. The barrel of the gun has an inner rod with a thumbscrew at the end. When the thumbscrew is turned inward as far as it will go, the gun barrel is closed. Turning the screw outward partly opens the outlet and controls the amount of powder emitted when the bulb is compressed. To produce a dense white cloud of powder, either as a steady stream or in small puffs, gently squeeze the rubber bulb.

Our first efforts with the Tel Tru Borozin A.C. Gun were not exactly stellar performances. We have to admit it took a bit of practice to get the powder dispersion just right and to create a clearly visible aerosol. More than once, we managed to direct clumps of powder onto carpeting and other horizontal surfaces. We also learned that to see the air-movement patterns under poor general lighting conditions (usually below 10 foot-candles), we had to illuminate the aerosolized powder with a bright light shining perpendicular to it. This sometimes called for three-handed operation. Perseverance paid off, however, and we now are so comfortable with this technology that we use it almost exclusively in our building evaluations. The one drawback is filling the gun--white powder everywhere. We have found, however, that the job can be neatly done with a demitasse or lab spoon in an area away from drafts. A natural-bristle makeup brush makes fast work of any spilled powder.

A Final Note

Before we leave this topic, we thought it best to mention yet another commonly used air-movement tracking system: smoke candles or smoke-generating matches. When lit, these systems produce a lot of visible smoke, but their application is limited. They will be the subject of a future article.

(*.) Available through Lab Safety Supply, or Professional Equipment.

(**.) E. Vernon Hill Inc., P.O. Box 1516, Benicia, CA 94510-4516.