• Temperature affects both water activity and microbial growth. Warmer temperatures promote rapid growth and many bacteria can still flourish at lower temperatures.
  • Sunlight, both UV and visible light, has been shown to alter the survival of microorganisms surviving in the built environment.
  • Moisture is one of the most potent contributors to microbial survival in air and on surfaces.
  • Low relative humidity (20-30%) increases the ability of microorganisms to become aerosolized from surfaces and suspended due to occupant traffic or disturbance.
  • High relative humidity (>80%) contributes to microbial survival and activity on surfaces.
  • Surface material is also critical; although all surfaces can function as a physical substrate, the chemical composition of the material provides a food source for the colonizing microorganisms and potentially selects for different species. Studies have demonstrated that cellulose-based surface materials, such as, for example, wood, can stimulate microbial growth more rapidly than inorganic materials, such as gypsum, mortar, and concrete.
  • pH is also important, as many metabolic processes are more energetically favorable at neutral pH; therefore, materials with an alkaline or acid pH can retard microbial growth.
  • Physical composition of the surface material will affect which organisms can access the surface. Even the surface roughness, porosity, and position in the environment (for example, the ceiling or the floor) can influence the dynamics of microbial colonization and growth. However, how these variables affect microbial metabolism and fungal–bacterial interactions remains to be elucidated and is an active area of research.
  • Biofilms can form on built environment surfaces, especially in moist areas such as sinks and bathroom showers, facilitating transfer through everyday activities.
  • Microenvironments within carpets can create pockets of high relative humidity that can aid in the growth, prolonged survival, and transfer of microorganisms to people.

How should the cleaning industry deal with large variability in local environmental conditions, including local weather, building materials, humidity, temperature, and indoor activities?

Distinguishing the living from the dead (or inactive)

The viability of microorganisms that exist in indoor environments is an area of great interest for the cleaning industry. Advances in techniques to visualize microorganisms in the air, in water, or on surfaces, and to determine their viability and activity, will significantly change how we clean. Are the bacteria multiplying, fungi or mold growing, or are the viruses surviving? Are the bacteria and fungi producing microbial volatile organic compounds (MVOCs) that can influence human health outcomes? Knowing the answers will help us ensure with what and how we clean has the least negative health consequences.

For example, dust is a rich, heterogeneous mixture of materials, providing plentiful substrate for microbial growth. When exposed to moisture, the resulting germination of fungal and bacterial spores or dormant cells leads to an increase in metabolic products, which can include chlorinated hydrocarbons, amines, terpenes, alcohols, aldehydes, and ketones, as well as sulfuric and aromatic compounds.

We know that microbial metabolic products can affect human health and cause nasopharyngeal inflammation, wheezing, cough, shortness of breath, onset and exacerbation of asthma, bronchitis, respiratory infections, allergic rhinitis, eczema, and other allergies.

Air and surfaces and infections

Everyone who enters built spaces has extensive interactions with the air and surfaces. These interactions have traditionally been examined only regarding the transmission of potential disease-causing germs. Microbial transmission between occupants and the built environment is reciprocal. For example, bacterial pathogens such as Bacillus anthracis, Legionella pneumophila, and Mycobacterium tuberculosis; fungal pathogens such as Cryptococcus neoformans, Histoplasma capsulatum, and Aspergillus fumigatus; and pathogenic viruses such as rhinovirus and influenza virus, can be transmitted by direct inhalation. Other pathogens, such as Clostridium difficile, Staphylococcus aureus, Pseudomonas aeruginosa, Pseudomonas putida, and Enterococcus faecalis, as well as norovirus and influenza virus, can be transmitted through surface contact. As people move throughout the built environment, microorganisms are constantly transferred.

Are all germs harmful?

Throughout history, most efforts to determine the influence of indoor microorganisms on health have focused primarily on the negative impact of disease and allergies. How would the way we clean change if we could shift our understanding of the microbiology of the built environment from a purely negative role (that is, causing disease) to combining a positive role (that is, protective or preventive)?

Germs have adapted to the built environment

The physical and chemical properties of buildings and the surface materials encountered by microorganisms in the built environment are quite different from materials and surfaces in the natural environment. Wood surfaces are often treated with chemicals to preserve them. Gypsum, fiberboard, drywall, synthetic carpets, and surface lacquers create environments unlike any other. Genomic sequencing and culture studies show that different surface chemistries and physical structures promote the growth of various germs. For example, shower curtains are mainly colonized by bacteria associated with Sphingomonas and Methylobacterium. Physical surfaces in buildings have been shown to be primary sites for bacterial adhesion and biofilm formation.

The cleaning industry is essential

The cleaning industry has a significant role in improving human health outcomes by cleaning for health. There is an immediate need by the cleaning industry to improve the measurement of cleanliness.

The COVID-19 pandemic led to a rapid expansion and the use of real-time sensors for indoor air quality. We need the development of real-time microbial sensors to detect exposures for individuals within the built environment that can be correlated with health and disease metrics. Our default approach is to attempt to make the built environment as hostile to microbial life as possible to prevent microbial infections.

Buildings are not isolation chambers. If we spend 90% of our life indoors, then there is a need to better understand why, when, and how microorganisms transfer from the built environment to occupants, when these transfers lead to disease, and when these interactions are beneficial to built environment occupants.