A new report based on a colloquium in 2015, gives a very useful – and broad – overview of how microbes in the built environment affect human health – both in causing disease (infections, allergies etc) and conferring essential health benefits.
Traditionally the public see the home environment as a “source” of potentially harmful microbes. The concern is to know “where these “germs” lurk” assuming that if they can eradiate these sources, the family will be protected. This new report suggests a very different perspective, showing that the home is largely a “vector” rather than a source of infection, where the microbes are shed or introduced from the primary sources – people, pets, and the environment outside the home. These organisms can survive and are spread inside the home largely via hands, surfaces and the air – such that we become exposed and infected. This stresses the importance of the targeted approach to hygiene which sees hygiene not as a “state”, but an activity. Activity is based on understanding the key routes (hands, surfaces etc) by which infection is transmitted around living environments, and targeting hygiene practices in the places and at the times that matter, most particularly those associated with activities such as food, respiratory, hand and toilet hygiene, healthcare etc to prevent pathogens from spreading.
That having been said however, there are some exceptions, where the built environment can be a source of infection. This includes building water systems where Legionella bacteria and other opportunistic plumbing pathogens can become established, thrive – and cause infection. Some of the closest associations between the indoor microbiome and poor health have been documented in moldy buildings that become a primary source of actinomycetes and fungi, including Penicillium, Aspergillus, and Cladosporium. Epidemiological studies have linked these environments to respiratory irritation, allergies, and asthma, as well as secondary respiratory infections caused by bacteria and viruses.
The report discusses how difficult it is to prove causal links between ill health and indoor microbiomes, because it is almost impossible to separate the effects of microbes from each other and from those of other stressors such as dust and nonmicrobial volatiles;
However – to keep this issue in perspective – it is important to recognize that – of the 1 to 10 million species of microbes on the planet, scientists estimate that only 1% cause disease. The remainder contribute to ecosystems in ways we are only beginning to understand. In the human gut, for example, there are microbes that are critical for digestion and produce vitamins, antimicrobials, and neurotransmitters. Thus, it is logical to ask whether and to what extent indoor microbial communities can produce conditions that can influence positive health outcomes in occupants.
The report touches on our growing understanding of the need for exposure to broad range of largely non-harmful microbes from our indoor and outdoor environment, and from other people and domestic animals, particularly important in early childhood, when the immune system is still developing. The report cites examples showing that possible links between immune dysregulation due to lack of microbial exposure and health outcomes, such as increased incidence of childhood asthma, allergic rhinitis, and obesity. It also mentions some of the public health strategies which could be important in sustaining a “healthy” microbiome like promoting natural childbirth, breast feeding and living in a home with a dog. However, although the report mentions work showing that microbial colonization of the home is impacted by cleaning, it does not give an opinion as to whether this activity might contribute to health effects of diminished microbial exposure (extent and particularly diversity of exposure). The report says only “In the process of cleaning, we remove dust and associated microbes as well as some of the substances on which they live. Chemicals, high temperatures, or UV radiation are used to disinfect surfaces. The more frequently and thoroughly we clean, the less opportunity for microbial build-up on materials and the lower the microbial biomass”.
Interestingly the report also says “scientists recognize that somewhat paradoxically, exposure to bacteria, viruses, and parasites can help us build immunity that enables us to combat infection” – but gives no data to back this up. It also says “The antimicrobial soaps, antimicrobial wipes, and antimicrobial surfaces that we often use, particularly those containing triclosan, may be doing us more harm than good” This statement however relates to possible direct effects of triclosan rather than possible effects on the microbiology of the built environment.
The report concludes “To increase our understanding of the “healthy” microbiome in the built environment, we must expand our epidemiologic knowledge. These studies are challenging—investigators need to understand the time, concentration, and route of exposure to microbes and their byproducts, as well as the dose response. It is also important to characterize building factors that may moderate disease so that associations between the built environment and disease can be elucidated. As we conduct these studies, it will be important to evaluate risk across socioeconomic groups, age groups, and regions and to bear in mind the energy and carbon costs associated with any change in building design and operation.”
The full report can be found at: http://academy.asm.org/images/stories/documents/FAQ_Microbiology_of_Built_Environments_Resize.pdf