A rapid review was undertaken to consider the evidence for human exposure to harmful microorganisms from indoor air and surfaces. Published information about these contaminants, as well as measures to control them, including building design and energy conservation, were included in this review. Information on domestic dwellings, office environments, and other non-industrial settings was assessed to determine the reported prevalence, persistence, and transmission of microorganisms in these settings. Environmental factors that influence indoor microbiological colonization were also included. The evidence strongly indicates that ventilation is the primary factor for controlling indoor dampness, helping to mitigate indoor mold colonization and the accumulation of other indoor contaminants, including infectious microorganisms. Although modern building airtightness, including retrofits of older builds, contributes to thermal comfort and building energy efficiency, this may also limit a building’s ventilation capacity. This in turn can potentially allow biological pollutants to accumulate, increasing the likelihood of harmful exposures and ill-health effects for building occupants. Effective building design and maintenance, which promote appropriate levels of air exchange for indoor spaces, are therefore important for the control of indoor moisture and microbiological contamination.

It is anticipated that by 2028 there will be a significant increase in the use of HWHP systems in Great Britain (GB). Such systems are considered a better, energy-efficient alternative to fossil fuel-based burners and furnaces, as they use electricity. There are concerns that these systems are susceptible to microbial contamination because they hold water at lower temperatures. In particular, the concern is regarding Legionella contamination, as it can potentially cause disease in the general public and those who are maintaining and replacing these systems. Therefore, this review was focused on understanding the potential risk posed by their increased use and maintenance requirements. This review was approached systematically but was not a full systematic review. There were 61 papers that were considered potentially relevant to the research questions. Of these, 40 papers were considered relevant to the topic of Legionella in HWHP and underwent full article assessment and data extraction. The remaining papers were considered useful for background information. The scope of this review established that Legionella are a known risk in hot water systems that can be carried over to HWHP systems, yet there is minimal evidence to suggest that the current control measures are being appropriately applied to reduce the risk of exposure. When considering countrywide legislation and guidance, it appears that the risk is considered lower in single- or multi-family homes that do not require a centralised system. This review included the assessment of information regarding the safety of working with HWHP systems with regards to maintenance and replacement. The authors found a lack of information regarding these safety concerns. This review is among the first to systematically evaluate the risks of Legionella contamination in HWHP systems.