This chapter describes safe work practices when using laboratory hoods, answers frequently asked questions about this important engineering control equipment, and references the Hood and Laboratory Ventilation Policy at UNC-Chapel Hill.

The University of North Carolina at Chapel Hill is committed to providing engineering controls that minimize exposure to hazardous materials to students, faculty and staff who work in laboratories. These controls include biological safety cabinets (Chapter 16) and laboratory hoods. Laboratory hoods are also known as “chemical hoods”, “fume hoods” or simply “hoods”.

If you believe your hood is not working properly, contact Environment, Health and Safety at 919-962-5507 and ask to have your hood evaluated. You must review the UNC Hood and Laboratory Ventilation Policy listed later in this Chapter if you are considering the installation of a new hood.

Laboratory hoods are only effective if the hood operates properly AND the user exhibits good work practices. Refer to the section entitled Laboratory Hood Work Practice Guidelines for more information.

Q: What do I do if my hood alarm activates?
A: The alarm on the laboratory chemical hood notifies you that the hood is not performing as desired, which could lead to overexposure to chemicals. If the alarm triggers, take the following steps:

If the alarm sounds due to a scheduled power outage and someone turns it off, post the hood as “Out of Service” (Appendix 17-A) until power is restored. Turn the alarm back on before conducting further work in the hood.
Q: Why does my alarm always go off when the laboratory door is open?
A: Please keep in mind that when doors are propped open, the airflow in the laboratory is affected and the hood may go into alarm. If you believe your alarm is too sensitive, notify EHS for a calibration assessment. Never tamper with the alarm by taping over openings.
Q: Can I use radioactive material in my laboratory hood?
A: The Radiation Safety Committee, appointed by the Chancellor, formulates radiation policies and procedures. Responsibility for carrying out these policies and procedures rests with the Radiation Safety Officer who directs the Radiation Safety Section of the Department of Environment, Health and Safety. Hoods must receive individual authorization by the Radiation Safety Section. EHS provides the radiation caution signs that indicate which hoods it has authorized for use with radioactive materials. Hoods must have this posting before you use radioactive materials in them.
Q: Can I do my virology/bacteriology work in my laboratory hood?
A: In general, virology and bacteria work shall not occur in a laboratory chemical hood. When working with cultures, use a biological safety cabinet. For information about biological safety cabinets at UNC, refer to Chapter 16: Biological Safety Cabinets.
Q: How do I make modifications to my existing hood or exhaust system?
A: Students, faculty, staff and Facilities Services personnel must not modify hoods by drilling, cutting or removing the hardware originally provided with the hoods. Such modifications are likely to degrade hood containment performance and result in hood leakage. Installing a standard latticework of “monkey” bars at the rear of the hood is an exception. The installer must follow the hood manufacturer’s recommendations when installing these support bars in the hood. EHS must review and approve in advance any other proposed hood modifications, and post-test following modification.

Do not add shelving to the hood, nor block the rear slots or front airfoil at any time. Ensure the sash and panels are in place before operating.

A. Do not work in a malfunctioning hood.

Figure 17.1
Figure 17.1. Left: Hood marked out of service until repairs are completed. Right: Example of EHS inspection sticker.

B. Check the EHS inspection sticker on the hood (usually on the sash) to ensure it has been inspected within the past 12 months. EHS measures the face velocity of all hoods annually, notes any deficiencies, and refers them to UNC Facilities Services for correction. Recommended face velocities are between 90-120 feet per minute (fpm).

C. Test the airflow alarm prior to using the hood to ensure it is operating properly.

Figure 17.2
Figure 17.2. Three common examples of hood airflow alarm devices.

D. Check the sash height.

  1. EHS affixes these stickers (right) to vertical-sash laboratory hoods to remind users not to work with the sash above 18″. Try to keep the sash closed unless you are setting up or actively using the hood.
  2. You can raise and lower a correctly operating hood sash smoothly and with minimal effort. If you have difficulty operating the sash, or you cannot lower it completely, contact EHS. Do not place equipment, cords, tubing, etc. so that you cannot lower the sash quickly and completely.
  3. The recommended best practice for a combination sash hood (horizontal sliding panels within a vertical sliding sash) relies on completely closing the vertical sash while working through the horizontal sliders. Regular use of the horizontal sliding panels with the vertical sliding sash closed reduces chemical exposure and reduces energy expense. The vertical sliding sash should only be open during set up, not while manipulating objects in the hood with reactions present.
Figure 17.3
Figure 17.3. Combination sash hoods. Left: Correct position of a combination sash while performing experiments. Right: Only raise the vertical sash when setting up experiments.

E. Work at least 6″ into the hood to keep chemicals and vapors from exiting.

F. Do not work with your head breaking the front plane of the hood! Sashes at the proper working height generally create a physical barrier between the operator’s head and the inside of the hood. Working with your head in the hood often means that the sash is too high, or that the horizontal panels are opened too wide on a combination sash hood.

Breathing Zone
Breathing Zone at different working locations inside the sash.
Figure 17.4
Figure 17.4. Laboratory worker with his head between the horizontal sashes on a combination sash hood.

G. Take steps to maximize containment.

  1. Place blocks under large equipment to allow air to flow underneath the equipment.
  2. Keep the work area and bottom baffles clear from clutter.

H. Use chemical storage cabinets for long-term storage, not your hood. Items in a hood will impede and disturb the exhaust airflow and potentially reduce or eliminate the safety factor.

I. Reduce cross drafts, foot traffic past the hood, and quick movements in and around the hood. The recommended 100 fpm for hood face velocity is only a little more than one mile per hour (1.14 mph; 1.83 kph). Other sources of air movement can easily overcome this.

J. Remove electrical units or other spark sources from the hood when flammable liquids or gases are present. Do not place power strips or surge protectors in the hood. Plug in all electrical equipment outside of the hood.

K. The use of a laboratory hood does not negate the University policy on eye protection. Eye protection is required for all faculty, staff, students, and visitors in laboratories during experimental procedures that could produce liquid or solid projectiles.

Several laboratories are equipped with snorkel ducts, which consist of a bell mouth and an articulated connection to the exhaust system (Figure 17.5). The main difference between your laboratory chemical hood and the snorkel is that the latter does not fully surround the reaction at the point of release. For this reason, snorkels are not a substitute for a laboratory hood when handling toxic chemicals. Snorkels are far less effective in capturing dusts, mists, or fumes, and can typically only capture contaminants released within 6 inches (15 cm) of the unit. Snorkels are extremely susceptible to cross drafts.

A good use for laboratory snorkels is the capture and removal of thermal updrafts from benchtop-heated processes, or as local ventilation for benchtop apparatuses such as gas chromatographs. Snorkels generally operate at 45 feet per minute (fpm).

Figure 17.5
Figure 17.5
The University of North Carolina at Chapel Hill (University) is committed to providing students, faculty and staff with engineering controls that minimize their exposure to hazardous materials. This policy addresses the installation, removal, change, and monitoring of ducted and non-ducted equipment used to control exposure to chemicals, toxins, radionuclides, and biohazard agents at the University. For the specifics found in the policy please see the EHS website.
Proceed to Chapter Eighteen