Biomedical Engineering Reference
In-Depth Information
2.5.1.2 Isolation and Containment
Isolation and containment involve the physical separation of a potential nanomaterial emission source
(e.g., a process, task, or equipment) from workers to minimize the risk of exposure. Separation can
be done by either locating or enclosing the emission source in an area separated from the workers,
or by isolating the workers in a controlled booth or room for remote operations or observations. As
shown earlier in Figure 2.1, operations involving easily dispersed, dry, or highly toxic nanomateri-
als need to be conducted in enclosures. For example, dry nanomaterials, should be handled inside
a glove box. A number of nanomaterials such as carbon black, silica fumes, titanium oxide, and
metal oxides are synthesized inside closed circuit reactors, which provide workers with an adequate
isolation from the direct exposure to nanomaterials. However, other safe procedures are required
when workers enter the enclosure for maintenance. It is noted that isolation and containment are
commonly coupled with ventilation in order to vent and remove the suspended nanomaterials.
2.5.1.3 Ventilation
There are two types of ventilation available for the control of airborne nanomaterials: dilution (or
general) ventilation (DV) and local exhaust ventilation (LEV). Typically provided by a building's
heating, ventilation, and air-conditioning systems, DV is the dilution of contaminated air with
uncontaminated air for the control of airborne hazards. Unlike LEV, the supply and exhaust of air
in DV occurs across a relatively large area, room, or building. As a result, DV has the following
limitations: (1) The contaminant quantity must be small enough, such that the airflow rate needed
for dilution is practical; (2) the contaminant concentration must be low enough, such that workers'
exposure is below the threshold limit; (3) the contaminant toxicity must be low; and (4) the genera-
tion of contaminant should be relatively uniform within the area or room. As such, DV alone is not
as effective or satisfactory for the control of health hazards as is LEV (ACGIH, 2010). Nevertheless,
a well-designed DV can supplement and enhance the effectiveness of LEV. OSHA laboratory stan-
dard specifics of 4-12 room air changes per hour (ACH) is normally adequate for DV in laboratories
where LEV is used as a primary method of control. To prevent the migration of nanomaterials into
nearby rooms or areas, the workplace should be kept under negative pressure. However, ACGIH
notes that the ACH is a poor basis for ventilation criteria in workplaces where hazards, heat, or odor
controls are required.
A LEV system is characterized by its enclosing or exterior hood that intends to capture and
remove contaminants in proximity or as close as possible to the emission source. Compared to
exterior hoods, in particular, the enclosing hoods are designed to “contain” the contaminants inside
the enclosure, operating under a negative pressure differential with respect to the worker's breathing
zone. As a result, enclosures are more protective and should be considered whenever possible for
working with nanomaterials. Examples of such enclosures include laboratory chemical hoods, glove
boxes, biological safety cabinets (BSCs), and powder handling enclosures. For more details on the
operation of the aforementioned enclosing hoods, readers are referred to the guidance provided by
the NRC (NRC, 2011) and NIOSH (NIOSH, 2012). A brief summary of these two guidance docu-
ments is provided as follows.
Laboratory chemical hoods are the most common types of LEV systems in laboratory settings.
They consist of an exhaust fan that draws the air into the hood, a moveable sash, airflow guiding
slots, and a work surface. It is important to note that, although they look alike, the negative-pres-
sure chemical hood should not be mistaken with the positive-pressure laminar flow, clean bench.
Clean benches are those that blow HEPA filter-treated air into the operator's face with the inten-
tion of protecting the product instead of the worker, and, thus, should not be used to control expo-
sures to nanomaterials or hazardous materials. In the case of chemical hoods, the entry of air into
the hood needs to be smooth, uniform, and with an appropriate face velocity to ensure its proper
performance. Depending on the supply of air distribution, required airflow ventilation rates are
recommended in the range of 60-100 cubic feet per minute per square feet (cfm/ft
2
) of open hood
