Ohe Centers for Disease Control and Prevention published Guidelines for Preventing the Transmission of Tuberculosis in Health-Care Settings, with Special Focus on HIV-Related Issues' in December 1990. Partially in response to recent reports of TB outbreaks and transmission of Mycobacterium tuberculosis in institutional settings,27 the Federal TB Task Force called for the update and revision of the CDC guidelines. On Oct. 28, 1994, the CDC published Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health Care Facilities, 1994.8 At an October 1992 public meeting about the 1990 TB infection control guidelines, the CDC received suggestions that infection control policies should be based, in part, on the level of risk in each local facility. As a consequence, the revised guidelines direct personnel in all health care facilities to conduct a TB risk assessment. This risk assessment will allow health care personnel to implement TB infection control programs appropriate to their facility's level of risk of M. tuberculosis transmission. It also will provide greater flexibility in adapting the recommended controls to a wide variety of health care facilities, such as dental settings, in which few or no patients with TB are examined or treated.
The guidelines emphasize basing infection control policies on a hierarchy of control measures, including administrative controls, engineering controls and personal respiratory protection (Table 1); developing, implementing and maintaining a written TB infection control plan based on a risk assessment; providing TB training, education, counseling and screening to health care workers (HCWs); evaluating TB infection control programs.
This article should assist dental workers (DWs) in conducting a risk assessment and in identifying TB infection control interventions appropriate to the level of risk for M. tuberculosis transmission in the dental facility.
Although the CDC guidelines are directed primarily at in-patient health care facilities, specific considerations for ambulatory care settings-such as dental offices-are discussed in a separate section [Section II.M.2.eI (see sidebar, "CDC Guidelines for Dental Care Settings," page 600) with appropriate references to other parts of the guidelines. In this article, references to specific sections of the CDC guidelines are indicated in brackets. The term "HCWs" refers to all persons working in health care settings who could be exposed to TB. The terms dental facility, dental setting and dental office are used interchangeably and refer to any location, whether a private practice or part of a larger institution, in which dental treatment is provided.

# TRANSMISSION AND PATHOGENESIS [SECTION LBD
TB is a clinical illness caused by the bacterium M. tuberculosis.

# SPECIAI REPORT-TABLE I
Effective prevention and control of M. tuberculosis is based on a clear understanding of how TB is transmitted, how infection becomes established and how infection progresses to clinical disease.
M. tuberculosis is spread through airborne particles, known as droplet nuclei, that can be generated when persons with pulmonary or laryngeal TB sneeze, cough, speak or sing. The particles are an estimated 1 to 5 microns in size, and normal air currents can keep them airborne for prolonged periods and spread them throughout a room or building.9 If a person inhales droplet nuclei containing TB bacilli, infection may begin if the bacilli reach the alveoli. Within two to 10 weeks, the body's immunologic response to TB bacilli usually prevents further multiplication and spread. This condition is referred to as latent TB infection.
Persons with latent TB infec-tion usually have a positive skin test reaction to tuberculin purified protein derivative (PPD), do not have active TB and cannot infect others. In about 90 percent of Americans infected with TB, the infection remains latent for life, with no progression to active TB. In the United States, active TB disease will develop in the first year or two after infection with M. tuberculosis in about 5 percent of people who have been recently infected. In another 5 percent, disease will develop later in life. The risk of developing active TB varies with age and immunologic status.10 In general, people not receiving treatment who have or who are suspected of having pulmonary or laryngeal TB should be considered infectious if they are coughing, are undergoing cough-inducing or aerosol-generating procedures or have sputum smears that are positive for acid-fast bacilli . Persons with extrapulmonary TB are not considered infectious unless they have concomitant pulmonary disease, nonpulmonary disease located in the respiratory tract or oral cavity or extrapulmonary disease that includes an open abscess or lesion. Coinfection of human immunodeficiency virus does not appear to affect the infectiousness of patients with TB.
Hierarchy of control measures. A facility's TB infection control program should be based on a hierarchy of control measures (Table 1) and should be appropriate for the facility's level of risk of M. tuberculosis transmission (Table 2). The first level in the hierarchy is the use of administrative controls, which are intended primarily to reduce the risk of exposing uninfected people to people who have infectious TB. These measures include developing and implementing policies and protocols for early identification and isolation of patients suspected of having active TB or, in facilities that do not have TB isolation capabilities, for referral to a collaborating facility that does have such capabilities. Other administrative measures include ensuring the use of effective work practices, performing PPD skin testing on HCWs and educating HCWs about TB.
The second level of the hierarchy includes engineering controls to prevent the spread and reduce the concentration of infectious droplet nuclei. These controls may include the use of local exhaust devices (such as booths or hoods), the dilution and removal of contaminated air via general ventilation, controlling the direction of airflow and cleaning air by filtration or-as an adjunct to ventilation-by ultraviolet germicidal In all dental settings, an initial or baseline risk assessment should be conducted by a designated person who understands the basic principles of infection control and can adapt TB infection control recommendations according to the facility's level of risk for M. tuberculosis transmission. In institutional dental settings, this person may be the hospital epidemiologist, an infection control practitioner or an occupational health specialist. In non-institutional dental settings, such as private dental offices, the risk assessment usually is conducted by the dentist or another staff member who has been assigned responsibility for the office's infection control program. Sources for training specific to TB infection control risk assessment may include the local public health department, state or local dental society, academic institutions, a local infection control organization or an infection control practitioner in a local hospital. The guidelines recommend that the risk assessment should be conducted for each area of the facility. Since most dental settings constitute an entire "area," a single risk assessment should be sufficient.
Risk categories. Each dental facility should be classified into one of five defined categories of risk for M. tuberculosis transmission: minimal, very low, low, intermediate or high (Table 2). This classification should be based on three factors. The first factor is the incidence of active TB in the community or county served by the facility. This information should be available from the local or county public health department. The second factor is the number of patients with active TB who received dental treatment in the facility during the preceding 12 months (this does not include patients who were promptly screened and referred). The third factor is possible transmission of M. tuberculosis in the facility as evidenced by PPD skin test conversions among DWs or evidence of transmission between patients in the dental setting.
Minimal risk. Most dental settings probably will fall into the minimalor very-low-risk categories. A dental setting at minimal risk is one that does not provide treatment to patients with active TB and is located in a community in which no TB cases have been reported during the previous 12 months.
i SPECIA DEPORT- Each dental office in the minimalor very-low-risk category should identify collaborating facilities that are capable of evaluating, managing and/or providing urgent dental treatment to patients with suspected or active TB. These facilities should be identified before DWs or patients are placed at risk of possible exposure to M. tuberculosis so that patients suspected of having TB may be referred promptly. The risk assessment should be used to determine the need for a DW skin-testing program and the appropriate frequency of such testing. Baseline PPD skin testing of all DWs, performed by trained personnel, is recommended at the beginning of employment for facilities in all risk categories except minimal risk. However, baseline PPD skin testing of DWs in the minimalrisk category may be advisable so that if an unprotected expo-