ARCHIVED - Housing conditions that serve as risk factors for tuberculosis infection and disease

Canada Communicable Disease Report
Volume 33 • ACS-9
1 October 2007

An Advisory Committee Statement (ACS)

Canadian Tuberculosis Committee*†

Preamble

The Canadian Tuberculosis Committee (CTC) provides the Public Health Agency of Canada (PHAC) with ongoing, timely and scientifically based advice on national strategies and priorities with respect to tuberculosis prevention and control in Canada. PHAC acknowledges that the advice and recommendations set out in this statement are based upon the best currently available scientific knowledge and medical practice. It is disseminating this document for information purposes to those involved in preventing tuberculosis among those with inadequate housing.

Introduction

Housing conditions are used as socio-economic indicators of health and well-being^1-4. Poor housing quality and overcrowding are associated with poverty, specific ethnic groups and increased susceptibility to disease^4-6. Crowding, poor air quality within homes as a result of inadequate ventilation, and the presence of mold and smoke contribute to poor respiratory health in general and have been implicated in the spread and/or outcome of tuberculosis (TB)^7-10.

According to the 2001 Canadian Census, First Nations¹, Inuit² and recent immigrants (foreign-born) have a disproportionately higher share of housing needs than other Canadians^11-14. They have the highest risk of living in houses that are overcrowded and in disrepair, and/or they live in houses that cost³ more than 30% of their before-tax household income^12-14. While the following statement concentrates on the implications for TB of inadequate housing in First Nations communities, the conclusions apply equally well to the Métis⁴, Inuit and immigrant Canadians with inadequate housing.

TB in First Nations populations on and off reserve

TB rates continue to be a major public health problem in Canada in First Nations, Métis, Inuit and foreign-born populations15,16. First Nations people living on reserves have an 8-10 times higher TB notification rate than do non-Aboriginal Canadians; they also have a higher than average household occupancy density and a poorer quality of housing than other Canadians10,12,17,18. It is not surprising, therefore, that TB rates are higher in Canada's First Nations populations than among Canadian-born, non-Aboriginal people19-21. Factors contributing to the high rates of TB among First Nations are the prevalence of latent infection, co-morbidities (including diabetes), substance abuse, genetic factors and socio-economic factors10,15-17. Socio-economic factors that have been implicated in health outcome include ethnicity, income, employment status, education, poverty and housing conditions2,14,21-23. Overcrowded houses and poor ventilation increase both the likelihood of exposure to Mycobacterium tuberculosis and progression to disease10,23-26.

TB transmission

TB infection is spread when an individual with active respiratory TB coughs or sneezes M. tuberculosis bacilli that become aerosolized droplets of less than 5 μm diameter. An increased density of droplet nuclei in the air leads to an increased risk of infection⁹. As the number of inhaled bacilli increases, so too does the risk that disease will develop in individuals after they have become infected. An individual with active pulmonary TB (smear-positive) who is sneezing or coughing vigorously and frequently will exhale 10⁶ contaminated droplets²⁷. Some, but not all, of the droplets will contain the M. tuberculosis bacilli⁹. The aerosolized droplets settle very slowly and can remain suspended in the air for many hours. Therefore, TB transmission occurs with greater prevalence in poorly ventilated and crowded spaces^3,9,23,28. A sputum smear-positive individual with pulmonary TB is four to six times more contagious than a smear-negative case²⁹. However sputum smear-negative, culture-positive patients with pulmonary TB are also infectious to others^30-32.

Environmental factors related to housing that may enhance the likelihood of TB transmission include the following³³:

1. exposure of susceptible individuals to an infectious person in a relatively small, enclosed space;
2. inadequate ventilation that results in either insufficient dilution or removal of infectious droplet nuclei;
3. recirculation of air containing infectious droplet nuclei;
4. duration of exposure; and
5. the susceptibility of the exposed person.

What housing factors might contribute to tuberculosis transmission?

Crowding

Crowding has been identified as both a risk factor for TB transmission^3,9 and a characteristic of First Nations housing both on and off reserve^{10,23,28,34,35}. In communities where persons with TB disease live, crowded housing leads to an increased risk in terms of exposure to M. tuberculosis. The risk of exposure is also increased if there is limited air movement in an enclosed space^9,35. Lienhardt summarizes a number of studies showing that crowding is a risk factor for infection and for increased risk of disease after infection³⁵.

In a Canadian study it was found that an increase of 0.1 persons per room (PPR) increased the risk of two or more cases of TB in a community by 40%10. Statistics Canada uses the measure of PPR to assess crowding in houses. PPR is calculated by dividing the number of persons living in a dwelling by the number of rooms18. A room is defined as an enclosed area within a dwelling that is finished and suitable for year-round living; the definition does not include bathrooms18. This statistic is not sensitive to the size of the house or the rooms, or to the composition of the household. The National Occupancy Standard (NOS) sets bedroom requirements as a measure of household crowding14. A house is not considered crowded if there are enough bedrooms for the number and composition of the inhabitants, defined as follows:

Manitoba Family Services and Housing further clarifies that, according to NOS, parents and children may not use the same bedroom³⁶. Analysis of the census data from 2001 shows that 10.3% of all on-reserve original households across Canada fell below the standard for sufficient bedrooms for the household composition compared with 1.4% for Canadian-born non-Aboriginal households¹². According to the Canada Mortgage and Housing Corporation (CMHC), 50% of First Nations housing on reserve fell below CMHC housing standards for suitability⁵ and adequacy⁶. Approximately 9,100 households (15%) were overcrowded, and 6,765 (23%) were in need of repair³⁷.

The overall average number of persons per room for First Nations people is 0.6 PPR, 20% higher than for the rest of the Canadian population³⁸. A comparison between the crowding in on- and off-reserve housing using the same census data showed that people in on-reserve houses were more than twice as likely to live in crowded conditions as those in non-reserve First Nations households¹². While these statistics clearly indicate a general trend towards overcrowding in First Nations communities, they in fact mask the severe overcrowding that is characteristic of some communities. Crowded housing in First Nations communities in the Prairie provinces and in communities in the Canadian Shield region (the central area of Canada associated with the subarctic and boreal forest regions) is a greater problem than in other regions of Canada^4,39.

The guidelines developed by the NOS and those used by Statistics Canada to assess crowding are not being adhered to in First Nations communities. Furthermore these guidelines may not be adequate for the control and/or prevention of infectious diseases in communities that have elevated disease loads. While ill health is an acknowledged consequence of crowded houses there is little research on the mechanics of the spread of respiratory as well as other communicable diseases in these houses. Logically, one would expect individuals who are sleeping in the same room with the index patient or those in continual close contact (e.g. mother and young children) in the house might be at the highest risk, although this has not been documented.

Inadequate ventilation

Transmission of M. tuberculosis bacteria to a non-infected person is more likely if there is poor ventilation. Beggs et al. noted that occupancy density, room volume and air change rate are all directly correlated with the number of new TB infections among persons who share airspace⁹. One air change occurs in a room when a quantity of air equal to the volume of the room is supplied and/or exhausted. Room ventilation is usually expressed in terms of air changes per hour^9,33. Inadequate air change rates, negative airflow and recirculation of air have been identified as an occupational hazard in hospitals with respect to TB transmission⁴⁰. Studies in hospitals and health care facilities have shown that poor ventilation design or construction have contributed to the transmission of infection, particularly among clinical personnel in patient rooms with fewer than two air changes per hour⁴⁰.

There are no studies that address ventilation in private houses in terms of TB transmission, but there are specifications for health care facilities that treat a spectrum of patients from suspected cases to highly infectious patients³³. Many individuals with infectious pulmonary TB start therapy in their homes, and a form of "home isolation" is instituted until the person is deemed no longer infectious. It is assumed that those who share their home have already been exposed to the organisms prior to the diagnosis and the start of therapy by the index case. Housing units, which have shared mechanical ventilation with other housing units, should not be used by infectious cases.

Factors that may inhibit increased ventilation in a house are the outdoor temperature, noise, comfort, energy costs, the condition of windows or doors, or cultural and personal habits. The presence of a wood-burning stove will affect the air quality in the house if the stove is not properly maintained and ventilated. If the windows and/or doors do not have screens that are in good repair the residents might choose to keep them closed. Windows and doors also might be kept closed during the night for reasons of safety or personal preference. The mere presence of doors and windows, therefore, should not be considered as evidence that they are opened to encourage natural ventilation.

Heat recovery ventilation (HRV) systems bring outside air into a house and remove an equivalent amount of stale, indoor air. The fresh air is then tempered and circulated throughout the house as opposed to the case with conventional heating systems, which make use of recirculated household air. An advantage to this system is that the amount of fresh air that is brought into the house can be regulated depending on household requirements. While HRV systems are not specifically designed to address the spread of TB, adequate ventilation as provided by these systems can help prevent the growth of mold in the house. It is possible to install the systems in existing houses by modifying the ductwork, but HRV is more typically recommended for new homes. The cost of installation of such systems and their maintenance are likely limiting factors to their widespread use in northern communities.

Ultraviolet germicidal irradiation (UVGI) has been used to minimize the number of airborne microbes^41,42. HRV systems can be equipped with UV lights to filter the air, but there are no good data yet on the efficacy of the systems in residential settings. Alternatively, UV lamps installed in the ducts of ventilation systems or as upper room lamps can be effective in removing aerosolized bacilli from the air. However, like the HRV systems, UVGI systems require considerable maintenance and monitoring. Although they may not be appropriate for use in homes, they should be considered for public buildings (e.g. schools, community centres, churches, nursing stations or airports).

Mold and tobacco smoke

Homes that have inadequate ventilation (either mechanical or natural) are often damp or have mold growth resulting from high humidity and condensation. The absence of adequate central heating and insulation is an important factor in the growth of mold in a house. Assessment of where the mold is in the house may be required. Mold could be considered a proxy indicator of inadequate ventilation; however, homes that have a great deal of air leakage may also have mold43. Inadequate ventilation is one of multiple factors that contribute to the development of mold in a home. Household humidity and encumbered space may also contribute to mold growth in a house.

Dampness and mold have not been directly linked with the acquisition of TB infection. However, they have been implicated in increased susceptibility to respiratory infection, asthma and allergies among Canadian children⁸. Dales et al. found an association between exposure to indoor fungal contamination and altered T-cell differentiation in children⁴³. Children living in houses with bedrooms contaminated with fungi had reduced CD4/CD8 ratios compared with children with less contaminated bedrooms. It has also been found that the presence of mold and fungi in homes is associated with suppressed T-cell production, which has been linked to slower recovery from TB^43,44.

A higher incidence of TB transmission to children has been associated with exposure to environmental tobacco smoke. Children who had contact with index cases who were smokers showed a higher infection rate than those in contact with index cases who were non-smokers45. It has been postulated that cigarette smoke may impair the pulmonary defence mechanism, resulting in airways that are more susceptible to infection; however, there are no published studies to support this hypothesis45. Alternatively, the link between smoking and TB may be a reflection of poorer health-related habits and socio-cultural behaviour.

Duration of exposure

The extent and persistence of contact with an infected person are the main environmental factors for the transmission of TB. Thus, transmission of TB occurs most frequently as a result of prolonged contact in enclosed environments with an infectious person. Persons who are at the greatest risk of exposure to TB are those who live and sleep in the same household as an infected person^29,45-47. An association has been confirmed between overnight cough frequency and increased transmission among household contacts^47,48. It should therefore be considered that those who share a bedroom or sleep area are at increased risk of infection.

Studies in industrialized countries show that more than half of all non-sleep activities for employed people between 18 and 64 years of age occur inside the house^49,50. It is reasonable, therefore, to conclude that children, their primary caregiver and the elderly, who spend even more time indoors, are at increased risk of infection.

TB transmission within households and between family members, particularly transmission from adults to children, significantly increases morbidity rates among children and adolescents in certain communities^51,52. TB transmission in families in which there are two or more new cases (apart from the index case) of TB, within a specified time period, are identified as microepidemics⁴⁶. In communities where the prevalence of TB is high, microepidemics may go unnoticed simply because the pattern is not apparent. In a large study of TB patients and their contacts, Vidal et al., classified the contacts as belonging to families with a microepidemic (Group A), families with one new case, (Group B) and families with no new cases within a 6-year period (Group C)⁴⁶. It was found that new TB cases more frequently (41%) came from families that had been characterized as having microepidemics (Group A) than from families in which only a single new case had occurred (Group B) (21%), and that it was a small number of families that generated the most new TB cases in the study of contacts. There was a high frequency (45%) of incorrectly treated household members during the 1-year period prior to diagnosis of the index case in families with a microepidemic, indicating a long-term presence of an actively diseased family member⁴⁶. Although early diagnosis of disease is an important goal for preventing TB transmission, it is not uncommon for there to be a delay of weeks or months before a diagnosis is made, during which time household contacts are at continued risk of infection²⁰.

The risk of infection from limited contact with a single case of TB is higher when intensive exposure to contaminated air occurs, such as in small, enclosed rooms or air space, for example, on airplanes⁹. The amount of shared air space for occupants of a small house is significantly higher than one would find in a larger house occupied by the same number of people. While a 1,000 sq ft. house (92.90 m²) with three bedrooms may be adequate for a healthy family of six people the amount of shared airspace in a house of this size may put family members at increased risk of infection. A measurement of the amount of space per individual (15.48 m²/person in the above example), in addition to Statistics Canada's calculation of person per room, may give a better sense of the risk of exposure, since room size can and does vary substantially.

Summary

1. Compared with the Canadian-born, non-Aboriginal population, some members of First Nations, Inuit and immigrant populations continue to show a substantially higher incidence of TB.
2. In many regions of Canada, housing conditions in selected populations within these three groups do not meet the National Occupancy Standard of the CMHC and are characterized by high household occupancy density, poor air quality and inadequate ventilation.
3. In populations that already suffer high rates of TB, crowded housing and poor ventilation increase the risk of transmission and progression to disease among those who share living space.

Conclusion

Steps need to be taken to address the issue of inadequate housing for populations in Canada that have a high burden of TB disease.

Acknowledgements

The authors acknowledge the members of the Canadian Tuberculosis Committee and the Provincial and Territorial Tuberculosis Programs for their contribution and participation in the Canadian Tuberculosis Reporting System:

• Alberta Health and Wellness, Disease Control and Prevention Branch
• Division of Tuberculosis Control, British Columbia
  Centre for Disease Control
• Manitoba Tuberculosis Control Program, Department of Health and Wellness
• New Brunswick Department of Health and Community Services
• Newfoundland and Labrador Department of Health and Social Service
• Government of Northwest Territories, Office of the Chief Medical Officer of Health
• Nova Scotia Department of Health
• Department of Health & Social Services, Government of Nunavut
• Vaccine Preventable Diseases and TB Control Unit, Ontario Ministry of Health and Long-Term Care
• Department of Health and Social Services, Prince Edward Island
• Direction de la Protection de la Santé Publique, Ministère de la Santé et des Services Sociaux, Québec
• Tuberculosis Control Program, Saskatchewan Health
• Department of Health and Social Services, Yukon
• Association of Medical Microbiology and Infectious Disease Canada
• Canadian Lung Association
• Canadian Public Health Laboratory Network
• Citizenship and Immigration Canada
• Correctional Service of Canada
• First Nations and Inuit Health Branch, Health Canada
• National Microbiology Laboratory, Public Health Agency of Canada
• Tuberculosis Prevention and Control, Public Health Agency of Canada

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*Members: Dr. R. Long (Chair); Dr. H. Akwar, Dr. M. Baikie, Ms. C. Case, Dr. E. Ellis (Executive Secretary), Dr. K. Elwood, Ms. D. Gaskell, Dr. B. Graham, Ms. C. Hemsley, Dr. V. Hoeppner (Past Chair), Dr. A. Kabani, Dr. B. Kawa (Past member), Dr. M. Lem, Ms. Joy Marshall, Dr. P. Orr, Ms. E. Randell, Dr. L. Scott, Dr. F. Stratton, Dr. L. Sweet, Dr. T. Tannenbaum, Dr. W. Wobeser.

†This statement was prepared by Dr. L. Larcombe and Dr. P. Orr. It has been approved by the Canadian Tuberculosis Committee.

¹First Nations - persons who have identified themselves as being North American Indian including "status" and "non-status" individuals. Status Indians are registered with the federal government as Indians, according to the terms of the Indian Act.

²Inuit - Aboriginal people in northern Canada, who live primarily in Nunavut, Northwest Territories, northern Quebec and northern Labrador. The word means "people" in the Inuit language - Inuktitut.

³Costs include the following:

• for renters, rent and any payments for electricity, fuel, water and other municipal services; and
• for owners, mortgage payments (principal and interest), property taxes, along with payments for electricity, fuel, water and other municipal services²¹.

⁴Métis - people of mixed First Nation and European ancestry who identify themselves as Métis, distinct from First Nations people, Inuit or non-Aboriginal people.

⁵According to CMHC "suitable dwellings have enough bedrooms for the size and make-up of resident households, according to National Occupancy Standard (NOS) requirements."¹²

⁶According to CMHC "adequate dwellings are those reported by their residents as not requiring any major repairs."¹²