CDC acknowledges (below, in a 2009 publication on page 45) that Ebola requires BSL-4 containment because it "pose[s] a high individual risk of aerosol-transmitted laboratory infections and life-threatening disease that is frequently fatal, for which there are no vaccines or treatments..."
Biosafety Level 4 is required for work with dangerous and exotic agents that pose a high individual risk of aerosol-transmitted laboratory infections and life-threatening disease that is frequently fatal, for which there are no vaccines or treatments, or a related agent with unknown risk of transmission. Agents with a close or identical antigenic relationship to agents requiring BSL-4 containment must be handled at this level until sufficient data are obtained either to confirm continued work at this level, or re-designate the level. Laboratory staff must have specific and thorough training in handling extremely hazardous infectious agents. Laboratory staff must understand the primary and secondary containment functions of standard and special practices, containment equipment, and laboratory design characteristics.
All laboratory staff and supervisors must be competent in handling agents and procedures requiring BSL-4 containment. The laboratory supervisor in accordance with institutional policies controls access to the laboratory.
There are two models for BSL-4 laboratories: (Neither exists in regular hospitals--Nass)
1. A Cabinet Laboratory—Manipulation of agents must be performed in a Class III BSC; and
2. A Suit Laboratory—Personnel must wear a positive pressure supplied air protective suit.BSL-4 cabinet and suit laboratories have special engineering and design features to prevent microorganisms from being disseminated into the environment.And for those needing additional confirmation, on page 251 of this same CDC publication, CDC lists the required level of bio-containment for each known arbovirus and hemorrhagic fever virus. Ebola is among a small group that requires BioSafety Level 4.
The Federation of American Scientists describes how a BSL-4 facility is designed and functions:
BSL-4, Biosafety Level 4
Required for work with dangerous and exotic agents which pose a high individual risk of life-threatening disease. The facility is either in a separate building or in a controlled area within a building, which is completely isolated from all other areas of the building. Walls, floors, and ceilings of the facility are constructed to form a sealed internal shell which facilitates fumigation and is animal and insect proof. A dedicated non-recirculating ventilation system is provided. The supply and exhaust components of the system are balanced to assure directional airflow from the area of least hazard to the area(s) of greatest potential hazard. Within work areas of the facility, all activities are confined to Class III biological safety cabinets, or Class II biological safety cabinets used with one-piece positive pressure personnel suits ventilated by a life support system. The Biosafety Level 4 laboratory has special engineering and design features to prevent microorganisms from being disseminated into the environment. Personnel enter and leave the facility only through the clothing change and shower rooms, and shower each time they leave the facility. Personal clothing is removed in the outer clothing change room and kept there. A specially designed suit area may be provided in the facility to provide personnel protection equivalent to that provided by Class III cabinets. The exhaust air from the suit area is filtered by two sets of HEPA filters installed in series. Supplies and materials needed in the facility are brought in by way of double-doored autoclave, fumigation chamber, or airlock, which is appropriately decontaminated between each use. Viruses assigned to Biosafety Level 4 include Crimean-Congo hemorrhagic fever, Ebola, Junin, Lassa fever, Machupo, Marburg, and tick-borne encephalitis virus complex (including Absettarov, Hanzalova, Hypr, Kumlinge, Kyasanur Forest disease, Omsk hemorrhagic fever, and Russian Spring-Summer encephalitis).BTW, since BSL-4 agents are considered potential biowarfare threats, BSL-4 labs (such as that at Rocky Mountain Laboratory in Hamilton, Montana) require an iris scan for entry and have armed guards, to prevent theft of microorganisms. Will Bellevue Hospital add these features?
I should have earlier linked to this excellent piece by David Willman (LA Times) that explores potential aerosol transmission, the problems identifying when people become infectious, and the limitations of airport screening.