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RETRACTED ARTICLE: Lichens reveal the quality of indoor air in Selangor, Malaysia

This article was retracted on 28 March 2023

This article has been updated



Indoor air quality (IAQ) is a concern in kindergartens as children spend much of their time there. Yet, there is a shortage of biological indicators needed for assessing IAQ. Thus, this study evaluated IAQ using transplanted lichen Usnea misaminensis as a biological indicator.


Lichen samples, collected from Bukit Larut, Perak, Malaysia, were exposed to indoor and outdoor environments in an urban area (Ummi Aiman Kindergarten) and a rural area (Ummi Qaseh Pelangi Kindergarten) for 2 months during August 15 to October 14, 2019. The concentrations of 12 selected elements and the vitality of the lichens were then evaluated.


Increased concentrations of eleven of the twelve elements deposited in the lichen samples in both urban and rural areas were observed. For both areas, the element concentrations in the samples from the indoor environment was lower than those from the outdoor environment, and those in the rural area were lower than those from in the urban area, suggesting the impacts of traffic emissions. The vitality of the lichens showed no significant change in indoor environment, compared to that in outdoor environment, indicating that even exposed to indoor environment, the lichens remained effective biological indicators as same as they were in the outdoor environment.


Lichens are effective biological indicators for both outdoor and indoor environments. Furthermore, outdoor emissions could influence IAQ, which could be problematic in densely populated areas such as kindergartens. Mitigation measures should be taken.


Surrounding air influences human health. A recent research conducted by the World Health Organization (WHO) revealed that 92% of the world’s population lives in areas of poor air quality (BreatheLife 2009). This study focused on outdoor air pollution, but the threats from indoor air are worrisome (Hellweg et al. 2009). This is particularly important given that 85% of people spend most time indoors (e.g., residences, schools, office buildings, places of worship, and restaurants), suggesting the need for broader surveys (Al horr et al. 2016).

Some hazardous compounds may occur in indoor air environments, such as VOCs, PAHs, NO2, CO, CO2, and heavy metals, many of which are derived from human activities, cleaning materials, furniture, heating, and the intrusion of pollutants from outdoor environments such as road pollution, industrial emissions, and many more. Such sources can lead to the accumulation of contaminants in indoor air and an increased risk of asthma, lung infections, allergies, and a very high propensity for chronic diseases such as cancer over a lifetime (World Health Organization 2010). Compared to the adults, children are more susceptible to indoor air quality (IAQ) because they absorb twice as much air from the indoor environment by volume relative to their weight. Therefore, it is necessary to control the conditions of the indoor environment involved (Ginsberg et al. 2005).

Several studies on indoor air quality in school and kindergarten environments have been conducted (Darus et al. 2012; Zwoździak et al. 2013; Chatziadiakou et al. 2015), and other studies have also shown that there is a relationship between indoor air quality and the safety of students and teachers in schools (Salameh et al. 2015; Lin et al. 2017). These studies highlighted the value of monitoring air quality in a school setting, with the main objective being to minimize the exposure of students to air pollution and to better understand the technique to be used to mitigate the decline in indoor air quality to establish a more favorable, healthy, and efficient school atmosphere and to ensure student and staff safety.

Biological testing using susceptible species such as lichens provides an early warning of contaminating effects on biological elements that cannot be assessed by non-living things (Loppi 2014). Various monitoring methods using life measures have been performed, such as detecting changes in morphology, distribution and life frequency, and analyzing pollutants that were absorbed (Abas and Awang 2017; Abas et al. 2018a; Abas et al. 2019; Zulaini et al. 2019). Lichens are organisms formed by the symbiotic relationship between a fungus (mycobiont) and algae or cyanobacteria (photobiont) (Abas et al. 2018b). Lichens are significant biological indicators thanks to their ability to absorb foreign particles from the air. This ability is due to the absence of a cuticle layer in the lichen’s thalli which act to filter out the particulate matter in the air in other living things. Normally, absorbed particulate matter or pollutants will inhibit the lichen’s biochemical activity and therefore affect its vitality (Bačkor and Loppi 2009; Abas et al. 2020).

Lichen has been used as a biological indicator of outdoor air quality, and many studies have been carried out (Abas and Awang 2017; Abas et al. 2018a; Abas et al. 2019; Zulaini et al. 2019). Nonetheless, the use of lichen to show the indoor air quality is fairly new and there have been only a few studies using this method (Canha et al. 2012; Canha et al. 2014; Paoli et al. 2019). No studies using lichen to assess air quality in an indoor environment had been carried out in Malaysia until 2019. The lichen monitoring system should incorporate instrumental monitoring techniques, where lichen can also be used to quantify the contaminants specified in the indoor air quality standards (Darus et al. 2012). The use of lichen can also provide an early warning about indoor air quality conditions by looking at its vitality (Mohamad and Latif 2013).

This study was conducted to assess indoor air quality (IAQ) using a lichen (Usnea misaminensis) transplanting technique as a biological indicator for the selected kindergarten indoor and outdoor environments, covering urban and rural areas in the Hulu Langat District, Selangor, Malaysia. To assess the IAQ in the chosen environments, contaminants such as heavy metals and lichen vitality were tracked and analyzed. This study hypothesizes that the outdoor lichen will have a high concentration of accumulated pollutants. So, without the source of contaminants in the indoor environment, the pollutants from the outdoor environments will also decide the IAQ. Thus, the aims of this study are (i) to measure the degree to which outdoor air pollution affects indoor air quality, (ii) to compare lichen vitality between outdoor and indoor environments, and (iii) to check the capacity of lichen (U. misaminensis) as an indoor biological indicator.

Materials and methods

Experimental design and sampling procedures

Lichen transplanting techniques were used in urban and rural areas of the district of Hulu Langat, Selangor, Malaysia. The Hulu Langat District covers 829.4 km2 in the southeastern part of Selangor (District Office, Hulu Langat District, 2019). Kajang, a major town in the Hulu Langat district, was selected to represent the urban area as a sampling location. Kajang is a busy town with a high-density population, extremely high flows of traffic, and regular industrial and commercial activities. Semenyih, situated at the district outskirts, was selected as the sampling location representing the rural area. Semenyih is an area with a moderate density population, uninterrupted traffic flows, and most importantly, a very low level of air pollution as well as the presence of the nearby Mount Nuang Forest Reserve area. Both areas have a similar climate throughout the year, with warm and humid weather, high rainfall, and temperatures between 28 and 36 °C.

To obtain the IAQ reading in the Kajang city area, this study selected Taska Ummu Aiman Kindergarten (2.9845° N, 101.8107° E) with 48 students and 6 workers to represent kindergartens in the urban area, while Ummi Pelangi Qaseh Kindergarten (2.9325° N, 101.8618° E) in Semenyih, with 53 students and 8 workers, was selected to represent the rural area (Fig. 1). To assess the impact of outdoor pollution on IAQ, the transplanted lichens were also placed in the outdoor environment (at the main gate) for both places.

Fig. 1
figure 1

Map of the sampling sites

The lichen of the species U. misaminensis (Vain.) Motyka (Voucher No. BL143/2009) is from a group of fruticose lichen which have a shrub-like structure (Din et al. 2010). The lichen was collected from the highlands area of Bukit Larut, Perak (4.8623° N, 100.7930° E) which is an area that is far from any possible air pollution. U. misaminensis from this area has been used for biological monitoring purposes and it has been found that the chemical concentration of the area provides an overview of an area free of air pollution (Abas et al. 2020).

The collected lichen was then cleaned to ensure that no foreign material could interfere with the data of this study. After that, the sample was washed three times with distilled water. Then, the lichen sample was put into a net bag specially designed for this study (Protano et al. 2017). The estimated weight of the lichen sample to put in the net bag was between 20 and 50 g. In all, a total of 20 bags of lichen were prepared and placed in both the outdoor and indoor environments of the two kindergartens (excluding 100 g of segregated and treated samples). Ten bags were placed in the Ummi Aiman Kindergarten (five bags in the classroom and five bags at the main gate), while the other ten were placed in the Ummi Pelangi Qaseh Kindergarten (five in the classroom and five at the main gate). All bags are placed in areas at a height of 1.5 to 3 m to ensure that exposure to pollutants was at an optimum level. The exposure duration of the lichen samples was 2 months (August 15 to October 14, 2019), which is the optimal period for the lichen to collect and store pollutants from the air into its system. Lichen samples were sprayed with distilled water regularly with the help of students and staff at both kindergartens to ensure that they were constantly wet and hydrated. Both kindergartens have natural ventilation systems (lots of windows and air space) that are only closed in case of heavy rain. After the exposure period expired, the samples were retrieved and stored in a freezer at − 18 °C until the analysis procedures of pollutants and their vitality were performed.

Analysis procedure of selected elements

In the laboratory, the lichen samples were analyzed using a light microscope to observe the presence of foreign matter and were then cleaned before further procedures. The thalli of U. misaminensis were selected for trace element analysis as lichen thalli have a high ability to absorb and store trace elements from the environment.

Lichen samples were prepared by removing them from their respective net bags. Twenty grams of samples was then extracted in plastic flasks using 3 mL 70% nitric acid (HNO3), 0.2 mL 60% hydrofluoric acid (HF), and 0.5 mL 30% hydrogen peroxide (H2O2) in a microwave decomposition system (Milestone Ethos 900) at 280 °C and pressure 797.7 psi. The concentrations of selected trace elements (Al, As, Cd, Cr, Cu, Fe, Pb, Sb, V, and Zn) and non-trace element (Ca and S) were measured using ICP-MS (Perkin Elmer Sciex, Elan 6100) and presented based on net weight (μg g−1 dw). This study also analyzed the non-trace element due to its negative effects when in higher concentration, towards indoor air quality and human health (Protano et al. 2017). The quality of the analysis was reviewed based on the IAEA-336 (Lichen) Standard Reference Material manual (Paoli et al. 2019).

Lichen’s vitality measurement

To compare the vitality between lichen samples that were placed in the outdoor and indoor environments, chlorophyll (Chl.) a fluorescent emission analysis was used. The performance of photosynthetic components of the lichen samples was evaluated and measured based on the main principle of photochemical quantum, that is FV/FM where FV = FMF0 is the fluorescent variable while FM and F0 are the maximum and minimum values of the Chl. a fluorescence. Moreover, the overall index of photosynthetic performance is calculated based on the Performance Index formula (PIABS). The samples were then reactivated for 24 h, sprayed using mineral water, and stored at 16 °C under ambient light (70 μmol m−2 s−1). After that, the samples were then sprayed again with water and kept in the dark without light for 10 min. The samples were then put under a saturated light for a few seconds (3000 μmol m−2 s−1) and the fluorescent emission was recorded. The measurements were performed using the Plant Efficiency Analyzer (Handy PEA, Hansatech Ltd., King’s Lynn, Norfolk, UK) (Protano et al. 2017).

Data analyses

Non-parametric analysis was used in this study using R Software (2016). For each experiment, the Mann-Whitney U test (p < 0.05) was used to determine the significance to which the degree of collectivity (or variation in photosynthetic parameters) compared to the control sample. This was to show the difference between samples that were placed at the same location and measured based on the environment in which they were placed (external and internal environments).

For a better understanding of trace element assemblage data in the lichen, this study used the EC (exposed-to-control) ratio developed by Frati et al. (2005). The proposed scale is < 0.25 for severe loss, 0.25–0.75 loss, 0.75–1.25 normal, 1.25–1.75 poor assemblage, and > 1.75 assemblage. The EC ratios were also used to investigate the trace elements with values > 1.25 to detect any possible contamination from the indoor environment.


Table 1 shows the concentrations of elements accumulated in lichen U. misaminensis, as well as the photosynthetic parameters (an indicator of vitality) after the lichen samples were exposed to the outdoor and indoor environment in urban and rural areas for 2 months. The outdoor environment showed higher concentrations compared to the indoor environment in both areas (urban and rural).

Table 1 Element concentrations (μg g−1) and photosynthetic parameters (potential quantum yield of primary photochemistry—FV/FM and performance index—PIABS) in the lichen Usnea minaminensis before and after indoor or outdoor exposure (average ± standard deviation (median)). As a first step, we checked whether a significant accumulation (or variation of photosynthetic parameters) occurred, and the values in bold are significantly higher than the control samples; in such cases, we checked whether samples in the same locality differ according to their indoor/outdoor exposure (pairs in italics) (Mann-Whitney U test, p < 0.05)

The EC (exposed to control) ratio, which is the difference in concentration between the elements before and after the exposure to their selected respective environments (Fig. 2), shows that there was a significant increase for 8 out of 12 elements (except Ca, Fe, S, and Zn) from outdoor to indoor environments for both urban and rural areas, and for 5 out of 12 (Al, As, Cd, Cu, and Sb) in indoor environments in rural areas.

Fig. 2
figure 2

Exposed to control (EC) ratios of samples transplanted outdoors and indoors in urban and rural areas. The dashed line indicates a significant accumulation (threshold EC > 1.25)

The vitality of the lichen was examined by comparing photosynthetic performance between each sample after exposure to their respective environments, and the results showed that there was no significant effect of the indoor and outdoor environments in urban and rural areas on the vitality of lichens (Table 1). It should also be noted that due to the high humidity during the exposure period, there was an increase in the photosynthetic performance index (PIABS) in each sample from the outdoor environments (p < 0.05).


The assessment of indoor air quality (IAQ) is important, especially when it involves children from, for example, a kindergarten. Based on several different case studies, some results contradict one another. Yang et al. (2009) reported that the amount of suspended particulate matter (PM10) in the indoor environment was higher than in the outdoor environment, with an I/O ratio (indoor/outdoor) of 1.43–2.06 depending on the type of room in the school, with the highest values found in the classroom, which was explained by the fact that these results are closely related to student activities, such as walking or running during recess. This activity causes PM10 to be re-dispersed to the environment. Similar results were reported by Almeida et al. (2011), who found the PM2.5–10 concentration in a classroom significantly exceeded the ambient level and suggested that students’ physical activity led to the re-dispersion of suspended particles. Studies in Poland have shown that the concentration of particulate matter in schools during the winter was higher in the outdoor environment, which also suggests that the children were the cause of increased particulate matter in the indoor environment (Zwoździak et al. 2013). However, in another study, Tippayawong et al. (2009) showed significant evidence that the accumulation of suspended particles in the indoor environment was due to penetration from the outdoor environment rather than the student’s activity.

This study focuses on the accumulation of trace elements with the assumption that the concentrations of these elements in lichen exposed to the indoor and outdoor environments of the kindergartens are a reflection of the conditions in and current state of the selected environment. The results indicated that Al, As, and Cd were present in the classrooms of urban and rural areas, while Cu and Sb were present only in urban classrooms. The high concentrations of Cd, Al, Sb, and Cu in urban classrooms are thought to be due to traffic pollution arising from motor vehicle emissions, as I/O ratios (indoor/outdoor) that are less than 1 indicate that this element comes from outdoor environmental pollution. We presumed that heavy metals’ accumulation in lichen samples was due to automobile traffic pollution. Based on previous studies in the urban area of Malaysia, there was a high correlation between heavy metals’ accumulation in lichen and automobile traffic (Abas and Awang 2017; Abas et al. 2018a). A similar pattern has been found in many other areas of the world, that is, that air pollution caused by heavy metal deposition generally tends to be higher in urban zones with more traffic, than in rural areas with less traffic (Benitez et al. 2019). All those heavy metals usually come from anthropogenic activities such as industrial activity and motor vehicles emission. In addition, in densely populated urban area, heavy metals are trapped by buildings and prevent it from flying away from the urban vicinity (Samsudin et al. 2013; Amil et al. 2016).

Protano et al. (2017) used transplanting techniques with Pseudovernia furfuracea lichen for 2 months in five schools in central Italy, one in a high population area, and four in rural areas. The study found that the concentrations of heavy metals (As, Cd, Cr, Cu, Hg, Ni, and Pb) and polycyclic aromatic hydrocarbons were high in urban areas, but an I/O ratio of > 1 was found only for Cd for urban areas and Hg in rural areas. The study showed similarities with our study where Cd, Al, Sb, and Cu had I/O ratios of < 1 and are considered to be at polluted levels due to motor vehicles from the outdoor environment.

Canha et al. (2012, 2014) conducted transplanting experiments in urban and rural areas of Portugal using the Flavoparmelia caperata foliose lichen, which also distinguishes between outdoor and indoor environments. The study found that there was an accumulation of several chemical elements in both environments and Ca was thought to be from the indoor environment, possibly from the chalk used on the blackboard. For this present study, Ca was not a significant element concentrated in the lichen, due to the use of marker pens instead of chalk for learning purposes, thus reducing the re-dispersion of dust in the classroom.

The use of living organisms (plants, moss, and lichen) within the IAQ assessment framework is relatively new and rarely applied (Rzepka et al. 2010; Canha et al. 2012; Vuković et al. 2014; Protano et al. 2017). Other than the indoor school environment, Vuković et al. (2014) also conducted a study of an indoor environment in a car garage in Belgrade, Serbia, using Sphagnum girgensohnii moss as a biological indicator. The study found that moss samples placed at the garage door had higher absorption than samples placed far within the garage. The low humidity in the garage impeded the ability of the moss to absorb air in its surroundings, thus causing a low rate of absorption in the indoor environment. A study by Paoli et al. (2019) showed that a sample of Evernia prunastri lichen that was placed in a smoker’s car for 2 months accumulated high amounts of heavy metals (Al, As, Cd, Cr, Cu, Ni, Pb, and Sb) and nicotine, while the exposure duration also modified the photosynthetic activity in the thalli.

Referring to the vitality of the lichen sample in this study, the parameter Chl. a fluorescence has shown that the lichen samples were not affected by the exposure to the indoor environment and remained fresh. It is important to note that the improvement in performance index (PIABS) detected in these samples was due to the humidity of the outdoor environment during the rainy season. Besides, the vitality of the samples in the outdoor environment for both locations was similar despite the significant concentrations of heavy metals. This finding is in line with the findings of studies conducted by Guttová et al. (2011) and Lackovičová et al. (2013) who reported that the improved quality of the outdoor environment was due to the decrease in heavy metal concentrations in the air and that the E. prunastri sample (exposed for 6 months) also showed good signs of vitality in most areas of the city.

All of the data and findings of this study indicate the importance of the vitality to the selected biological indicators. Generally, samples in the indoor environment that are exposed to artificial light, ventilation systems, and a lack of air humidity should be seriously monitored. Therefore, this study provided sufficient hydration to the samples of the lichen in the internal environment and it was proven that the vitality of the lichen samples in the internal environment was similar to those in the external environment, giving the information that with the proper method and materials, lichen can act as a biological indicator for indoor environment air quality.


Lichen U. misaminensis was used as a biological indicator for assessing indoor air quality (IAQ) in kindergartens in urban and rural areas in the Hulu Langat District. This study focused on trace metal concentration and assumed that the trace metal concentration in the lichen’s thalli gave an overview of the quality of an environment. EC ratios indicated that the trace metal concentrations of the samples were high in urban environments. However, the concentration in the indoor environment was approximately the same for urban and rural areas, irrelated to the outdoor environment, and showed that the movement of pollutants from the outdoor environment into the indoor environment was limited. The particulate matter that was present in the indoor environment comprised elements that are closely related to traffic pollution (Al, As, Cd, Cu, and Sb). The vitality of the lichen that was exposed to the indoor environment was similar to that of in the outdoor environment, indicating that the lichen is suitable for the IAQ evaluation framework when monitoring trace elements using biological indicators. A 2-month exposure of lichen samples was sufficient to allow the trace elements to accumulate in the thalli and also to prevent any morphological damage to the lichen. Indoor pollution in kindergartens is mainly sourced from the outdoor environment. Therefore, prompt actions should be taken by authorities to confront this problem.

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We would like to acknowledge National Institute of Occupational Safety and Health (NIOSH), Universiti Kebangsaan Malaysia, and Universiti Tun Hussein Onn Malaysia for providing the facilities to conduct this study. We also want to thank Dr Rose Norman for her proofreading service for this article.


This study has been supported by Universiti Kebangsaan Malaysia through a research grant (GUP-2018-032).

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AA wrote the paper, analyzed the data, and conducted the experiment. SMM prepared the research design and analyzed the data. MTL, KA, and MSMN revised the analyzed data and manuscript. NM helped in providing the research facilities. The authors read and approved the final manuscript.

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Correspondence to Azlan Abas.

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Abas, A., Mazlan, S.M., Latif, M.T. et al. RETRACTED ARTICLE: Lichens reveal the quality of indoor air in Selangor, Malaysia. Ecol Process 10, 3 (2021).

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