# Mdpj _sciences_
## 7 Limitations and Recommendations  
After examining the articles and discussing the state-of-the-art topics, several limitations and gaps in the current scientific publications on vertical green systems for pollutant mitigation have been identified. One significant limitation is the lack of detailed information on the maintenance of green walls. Most of the selected studies do not address this topic, with the exception of [PERSON] et al. [17], which only mentions the maintenance frequency of the active green wall, and [PERSON] et al. [51], which provides reflections on the importance of maintaining green wall plants and practical suggestions for their management, including a predictive vegetation replacement system. Maintenance is known to be one of the most significant costs in green infrastructure design, alongside installation. Designing a robust maintenance system is crucial for the long-term performance of the plants, as prolonged exposure to high pollutant concentrations can reduce their capacity to absorb harmful compounds and may even lead to premature termination of their life cycle. Besides botanical care, it is also essential to regularly check the technologies that comprise the entire system, such as irrigation devices or materials within the layers (e.g., fabric layers or growth substrates), as these elements contribute to achieving optimal performance. In light of these observations, future research should focus on designing an effective maintenance program alongside the technological framework. Defining maintenance practices that ensure sustained performance over time is essential. Another gap found in the literature is the potential for captured pollutants to be re-released into the environment after impacting the leaves, a phenomenon known as pollutant resuspension. This re-emission can occur naturally due to wind or be triggered by rainfall washing the pollutants off the leaves. Only two studies address this issue: [PERSON] et al. [21], which specifically aims to determine the influence of rain on the remobilization of PM accumulated on leaves, and [PERSON] et al. [27], where the topic is only briefly mentioned. It is crucial for future research on green walls for pollutant capture to also evaluate the possibility of re-releasing these sequestered particles back into the environment. Therefore, it will be important to design systems that minimize resuspension by selecting appropriate materials or plants or by developing strategies to reduce this undesirable phenomenon. In light of the results obtained from the selected articles, this review proposes a strategy for the design and performance analysis of a green system for pollutant mitigation, offering insights for future research. Regarding the type of technology, it has been shown that airflow through vegetation substantially increases efficiency [26; 28; 29; 34; 36]. However, no significant differences in pollutant capture were found between modular systems on fixed supports, freestanding modular systems, or container systems. Therefore, the choice of this characteristic can depend solely on the installation site conditions. It is important to maintain a high water content as it helps retain particles better and makes vegetation more tolerant [19; 26; 55]. The most effective substrate identified is coconut husk [10; 26; 29]. The system identified by this review as potentially highly performant, and recommended for future experiments, is an active green wall. This system should have a fan positioned on the rear side and an automatic irrigation system that activates when a low substrate humidity value is detected. The substrate could be composed of a mix of coconut husk and organic material. The choice of technology should consider ease of maintenance, such as using lightweight tanks and pots for planting the most efficient plant species like C. _comosum_ and _S. trifascita_[28; 29; 57].