Use of Moss Bags as Active Biomonitors for Ambient Air Quality—Practices, Variability, and the Current State of Knowledge: A Scoping Review and Call for Future Research

Active biomonitoring using moss bags provides a cost-effective alternative to traditional sensor-based air quality networks, directly supporting multiple UN Sustainable Development Goals, including SDGs 3, 7, 11, and 13. However, the reliability of moss bag outputs is frequently challenged due to the wide-ranging variability in moss bag construction and exposure. To critically evaluate these disparities, this scoping review employed the PRISMA framework to synthesize 137 moss bag studies between 2005 and 2025 to assess the dominant moss bag parameter choices. The results revealed a ubiquitous lack of a “standardized operating protocol (SOP)” in almost every aspect of moss bag studies, which restricts the replicability of experiments. The preparatory stages revealed widespread “customization” of moss sampling methods; the use of water as the main moss cleaning–washing medium (~87% of the literature reviewed herein); drying at 105 °C (~80.7%); and a general preference for the Sphagnum species (74%). Regarding the constructional phases, preferences for cylindrical bag shapes (60% of the literature reviewed herein) and nylon mesh for the bag material (68%) were noted. However, the mesh aperture sizes (<1 to >10 mm2) and bag exposure duration (<4 to >10 weeks) exhibited ranges far too broad for cross-study replication. Less than 10% of the literature explicitly mentioned choices of moss packing density, bag elevation, irrigation, shading, and orientation. Reflections on the influences of environmental/meteorological variables on moss bag results were sparse. Less than 5.5% of the literature delved into any process-level discourses for parameter choices, while barely 2.8% of the literature attempted to establish SOPs for moss bags. Overall, this study highlights that much remains to be learned about the moss–pollutant interaction pathways, phytotoxicity, and chlorophyll damage, and that moss bag studies still need to be streamlined; they still lack clarity in their reporting of study designs/parameterization choices, which could collectively limit the future use of moss bags to support air quality policy and decision-making.