Medicinal Plants Threatened by Undocumented Emerging Pollutants: The Sub-Saharan African Viewpoint

Abstract

The history of medicinal plants on the African continent is huge, the oldest and probably the most diverse, for there are thousands of spoken languages, in the sub-Saharan African region, that are used during the traditional practices that utilize medicinal plants for healing purposes. However, our lines of research have exhibited a potential unprecedented threat to this remarkable history of African medicinal plants by emerging pollutants, the per- and polyfluoroalkyl substances (PFASs), which are yet to be efficiently and sufficiently reported and documented on in this region. Accordingly, this review chapter reports on sub-Saharan African medicinal plants with the aim of highlighting how undocumented PFASs, in this region, present a huge threat to the extraordinary diversity of these plants and the therapy that they have assisted the low-income populations of this region with for centuries. Thus, we recommend appropriate and regular assessments and monitoring of PFASs, particularly perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) the most studied of these substances and their substitutes, in medicinal plants of the region, for these chemicals have been scientifically proven to be associated to numerous health concerns. The region should also consider properly regulating these compounds.

1. Introduction

Medicinal plants have been in use since ancient times [1]; they carry a long history [2]. These plants are an important mode of combat to serious illnesses and diseases in the world [3]. These crops and their derivatives are used for healing by various populations, and in extreme scenarios, these plants have been chosen as natural alternatives or substitutes to their orthodox counterparts [4]. Reported evidence has indicated that these natural products and their derivatives account for an estimated more than 50% of all the drugs used globally [5]. Available data have previously estimated that 90% of world’s rural people use medicinal plants for therapeutic purposes, and according to a recent survey by the World Health Organisation (WHO) 87% of its African member states population rely on traditional medicine, mainly medicinal plants, as their main primary health care source [6, 7, 8, 9]. For instance, it has been reported that 90% of the Ethiopians use herbal remedies as their main source of medicines, while up to 80% of South Africans are estimated to be in consultation with healers traditional [6, 7]. Thus, sub-Saharan African medicinal plants in their diverse forms are holistic involving both the body and the mind [6].

Obviously, the history of medicinal plants on the African continent is huge, the oldest, and probably the most diverse [6], for there are over 2000 spoken languages [10], in the sub-Saharan African region, during the use of traditional medicinal plants for healing purposes. In this regard, a variety of medicinal plants is reported to be used for the treatment of ailments in this region of Africa [11]. Hence, it has been reported that sub-Saharan Africa alone has over 50,000 distinct plant species, of which more than 25% of these species is reported to have been used for several centuries in traditional African medicine for the prevention and treatment of illnesses [12]. Recent reports have suggested that the remarkable and enormous biodiversity in medicinal plants in sub-Saharan Africa should not be surprising considering that the continent is geographically located within the tropical and subtropical climate [6, 12, 13]. The region has one of the biggest forests of the world, the Congo basin, which spans across six countries, namely the entire Central African Region (Figure 1), i.e., Cameroon, Central African Republic, Democratic Republic of the Congo (DRC), Republic of the Congo, Equatorial Guinea and Gabon. The basin on its own is estimated to have approximately 10,000 species of tropical plants and 30% of these are endemic to the region. This forest, in some extents, provides livelihood to millions of people across this region.

Figure 1.

Sub-Saharan African regions.

Nevertheless, despite the vast medicinal plants’ diversity and highest endemism, sub-Saharan Africa still doesn’t have sufficient drugs being commercialized globally [6, 14, 15]. This has been exacerbated by the fact that only a small fraction of medicinal plants, on the African continent, is from commercial cultivated sources, as most of medicinal plants consumed in sub-Saharan Africa and those destined for exportation are mostly harvested from the wild, including forests and national parks; albeit few countries, including Madagascar, Kenya and South Africa, taking initiatives towards commercially producing medicinal plants [16]. Therefore, we can only hope for the continent to efficiently making use of its remarkable medicinal plant potentials to improve the lives of its growing population. Hence, there are positive signs in this regard which have emerged recently. For instance, a WHO reported that by 2018, more than 85% of the total Member States in the WHO African Region have reported having a national policy for medicinal plants and others, compared to Western Pacific Region and the Eastern Mediterranean Region WHO Member States with 65% and 45%, respectively [8].

Additionally, the same report also found that the African region scored the highest percentage (>80%) of countries with national or state level laws and regulations for medicinal plants and others. Certainly, this is a promising path that the African continent has embarked on, even though more still need to be established.

Furthermore, there are several other threats to sub-Saharan Africa’s medicinal plant potentials. For example, the literature reviewed has indicated that medicinal plants on the continent are disappearing due to the destruction of its natural habitats in the form of high rates of deforestation, rapid agricultural development, urbanization, and uncontrolled harvesting of these plant materials [6, 7, 12, 17, 18]. Nonetheless, there are threats that have emerged during the last few decades, and which, in our view, have not been reported on, and have thus remained undocumented. They are threats form emerging persistent organic pollutants (POPs), perfluoroalkyl and polyfluoroalkyl substances (PFASs), in particular. Emerging contaminants are contaminants about which we have a new awareness or understanding about how they move in the environment or affect health [19].

2. Persistent organic pollutants: definition and environmental fates

Several international environmental organizations, including the Geneva Inter-Organisation Programme for the Sound Management of Chemicals, the United Nations (UN) under its United Nations Environmental Programme (UNEP), the Food and Agriculture Organization (FAO) and the WHO, have described POPs as chemicals that are stable and persist in the environment, bioaccumulate in organisms and the food chain, are toxic to humans as well as animals, and have chronic effects such as the disruption of reproductive, immune and endocrine systems, as well as being carcinogenic [20, 21, 22, 23, 24]. It is believed that these substances can enter the environment through several ways, including release from waste dumps, spillages, industrial and agricultural waste, urban/agricultural runoff and the burning of various materials, thus being distributed in various environmental matrices, including water, air, soils, sediments and living organisms [25, 26, 27, 28]. Given the fact that POPs have bioaccumulation potentials and can travel long distances to places far from the points of release by means of waterways, atmospheric exchange and agricultural runoffs, POPs have been detected even in pristine areas such as the Antarctica and the Arctic regions, regions with minimum direct anthropogenic disturbance [25, 28, 29]. In 1997, in order to limit POPs transportation and environmental contamination, the international community decided to work towards the establishment of a convention that would serve as an international, legally binding instrument, to reduce and/or eliminate the release of POPs, as identified in the UNEP Governing Council Decision 19/13C [20]. Consequently, under the Stockholm Convention of POPs (COP-4) for global action, the UNEP, in 1995, listed twelve POPs which are also known as the “dirty dozen”, and consisted of Aldrin, dieldrin, dichloro-diphenyl-trichloroethane (DDT), endrin, heptachlor, chlordane, hexachlorobenzene (HCB), mirex, toxaphene, PCDD/Fs i.e., polychlorinated dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs). In this regard, the Convention’s Governing Council took a decision 18/32 to begin investigating POPs, and their persistence in the environment, and thus initiate the eradication or restricted use of these substances, and ultimately minimize the contamination of food chain [28, 29, 30, 31, 32].

Moreover, the sources of different POPs have been established. For instance, deliberate application to crops and soils is suggested to be the source of agrochemical POPs, while organochlorine pesticides (OCPs) and other industrial chemicals are reported to be intentionally produced for various uses, for example as flame retardants, as ingredients in consumer products, including electronic goods, which generally result in their unintentional release into the environment, some in the form of e-waste [28, 33, 34, 35, 36].

In addition, the use of fire-fighting foams, vehicles and the burning of wood, have been mentioned as potential contributors to POP release into the environment in developing countries [28, 35]. Organic pollutants can enter the coastal environment by several processes and once introduced to this environment, they are subject to biogeochemical cycling, sinking, and other environmental processes [28, 37].

Furthermore, evidence suggests that the low rate of escape of POPs into water reservoirs (i.e., streams), or stock of materials and products, is a source of great concern because it could result in exposure that could cause subtle toxicological effects in humans and biota [38, 39, 40, 41]. Similarly, an increasing number of materials containing POPs, are used in building materials, in goods and in various consumer products [42, 43]. For POPs contained in consumer products, their low vapor pressure can result in a slow but significant release into the environment [35] which can come from direct volatilization as well as microscale abrasion of plastics [42, 43]. Following release, the fate of the POP compound in the environment is largely based on its physico-chemical properties and the characteristics of the environment [44].

Besides, it has been suggested that the process of environmental transport of these compounds and their detection into food supplies will be augmented if the compound is in the biosolids applied to agricultural lands, in wastewater effluents discharged to surface waters and in landfills adjacent to agricultural lands, and if industrial facilities that use the compound are located near sources of food [45]. The exposure of infants has also been reported as it becomes evident that POPs can be transferred from mother to infant via breast milk, and umbilical cord serum [46, 47, 48, 49].

In addition, regardless of the tremendous work that has been done on the African continent regarding reporting and documenting the prevalence of POPs in the African environment over the years [30, 50, 51, 52, 53], there is not sufficient evidence on the state of emerging pollutants, such as PFASs, on the continent, compared to the rest of the world; and the reviewed literature has suggested that these undocumented pollutants are a threat to the general African environment [54].

2.1 What are per- and polyfluoroalkyl substances?

There is no general accepted definition of PFASs. However, PFASs are chemicals that fall under the category of new emerging pollutants, for they exhibit properties which are different from traditional pollutants [28] and were anthropogenically synthesized since 1950s by linking a chain of carbon and fluorine atoms together using two major manufacturing methods, namely electrochemical fluorination (ECF) and telomerization technics [55, 56, 57, 58, 59, 60]. PFASs are therefore not present in the environment naturally, but are referred to as “forever chemicals”, unlike its counterparts such as heavy metals, e.g., compounds such as arsenic (As), mercury (Hg), lead (Pb), cadmium (Cd), chromium (Cr), etc.

These industrial chemicals (i.e. PFASs) contain at least one perfluoroalkyl fraction and have been previously referred to as “perfluorinated chemicals” (PFCs); albeit “PFCs” being a term describing perfluorocarbons, i.e., substances that contain only carbon and fluorine atoms, and having physical properties, such as being oil and water repellent and temperature resistant and reducing friction, and unique chemical functionalities that are fundamentally different from those of many PFASs [59, 60]. Because of these attributes, PFASs have been largely used as part of feedstocks in several manufacturing processes to make consumer and industrial products [60]. Accordingly, the common uses of PFASs have included: (a) non-stick cookware, stain resistant carpets and fabrics, (b) coatings on some food packaging (e.g., microwave popcorn bags and fast-food wrappers), (c) components of fire-fighting foam, (d) many industrial applications, (e) consumer products—for example, products that are stain and/or water resistant, cosmetics, and some cleaning products [19, 60].

Additionally, a common terminology for the nomenclature of PFASs has thus been agreed upon and saw PFASs divided in two classes, namely non-polymeric and polymeric PFASs [60], each with subclasses, groups and subgroups, as depicted on Table 1. For more details, there are various references therein [59, 60, 61, 62, 63, 64]. In addition, it is currently estimated that more than 5000 known PFAS chemicals exist, with perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) being the most manufactured; and their numbers are expected to increase as industries continue to invent and manufacture new substances [61, 62]. For instance, in 2006, it was reported that the production of these chemicals reached its peak in China, a major trading sub–Saharan African partner, with more than 250 tons/year [58].

Per- and polyfluoroalkyl substances (PFASs)
Non-polymers		Polymers
Perfluoroalkyl substances╬	Polyfluoroalkyl substances╣
Perfluoroalkyl acids (PFAAs)a	Fluorotelomer-based substancesc	Fluoropolymers
Perfluoroalkyl carboxylic acids/perfluoroalkyl carboxylates (PFCAs)a	Perfluoroalkane sulfonamido Substancesb	Polymeric perfluoropolyethers (PFPE)
Perfluoroalkane sulfonic acids/perfluoroalkane sulfonates (PFSAs)b	Polyfluoroalkyl ether acidsc	Side-chain fluorinated polymers
Perfluoroalkane sulfonamides (FASAs)b

Table 1.

Per- and polyfluoroalkyl substances family.

^╬

All hydrogen (H) atoms on all carbon (C) atoms in alkyl chain attached to a functional group have been replaced with fluorine (F).

^╣

All H atoms on at least one (but not all) C atoms have been replaced with F.

^a

Manufactured by either ECF or telomerization.

^b

Manufactured by ECF.

^c

Manufactured by telomerization.

Adapted from [60].

2.2 Reasons behind PFASs attention and how people get exposed to them

During the last decades, there has been sufficient evidence on the distribution of PFASs in the environment at large, predominantly in the aquatic and biota environments, but most importantly the attention has been on the fact that these substances have been said to be capable of widely spreading [65]. In this regard, the discovery of PFASs in serum, urine and other tissue samples has prompted researchers wanting to know whether these chemicals can lead to health issues [66]. Similarly, studies have found that PFASs get exposed to in various exposure pathways, including the numerous products to which the application of PFASs led to their manufacturing, thus causing multiple opportunities for exposure. On the other hand, there are already more than 5000 of these compounds available worldwide, and their number is expected to increase. In addition, PFAS’s unique properties have also made them stable in the environment and food sources, ultimately making them to be persistent, i.e., that once they enter the environment PFAS remain in the it for an unknown length of period and take many years to leave the body they have entered [66, 67, 68]. The other subject of PFAS’s attention is their bioaccumulation characteristic [65].

Moreover, it has been argued that all sources of exposure are not conclusively understood [65], but numerous studies have suggested that people are most likely to be exposed to these compounds by means of drinking and consuming PFAS-contaminated water or food, using products made with PFAS, or breathing air containing PFAS [69, 70, 71, 72, 73, 74, 75].

The next pressing subjects of attention as far as PFASs are concerned are PFAS precursors (pre-PFAS) and alternatives to PFASs of concerns. Hence, pre-PFASs are formed by means of biotic or abiotic degradation from other PFASs [60, 76], while concerns over the effect of PFASs, mostly PFOA and PFOS, on humans and the environment led to an interest in exploring suitable alternatives to these substances; and ultimately three types of alternatives to PFASs, namely, (i) substances with shorter per- or polyfluorinated carbon chains, (ii) non-fluorine-containing substances and (iii) non-chemical techniques. Further details on some of the commonly known commercial alternatives PFASs and their potential health impacts are available [76, 77, 78]. Short-chain PFASs refer to those with five and seven or fewer carbons that are perfluorinated, while long-chain have six and eight or more perfluorinated carbons [77]. Concerns over PFAS alternatives are to be exacerbated by the expansion of world’s biggest economies who are continuously manufacturing these chemicals in hundreds of tons per annum. Examples of commonly known and commercially available PFAS alternatives to long-chain PFASs, and which safety has been questioned are available in the literature, for example, see [76].

Moreover, like all other substances that are bioaccumulative, persistent and toxic in nature, PFASs have been reported to have the potential to cause health problems. As such, epidemiological evidence has suggested associations between perfluoroalkyl exposure and several health outcomes in humans and animals, even though cause-and-effect relationships for humans’ cases have remained inconclusive, which have implied that more studies are still needed. There are further details on the toxicological profile of perfluoroalkyls [79].

2.3 Prevalence and bioaccumulation of PFASs in plants

PFASs are highly soluble in water, a characteristic that make them to be easily absorbed and translocated in plants. This has become a great centre of interest for researchers wanting to comprehend the phytotoxicity of PFASs. Subsequently, during the recent years, there has been an increase in studies that investigate the prevalence and bioaccumulation of PFASs by plants, including cereals, fruits and vegetables. Thus, high concentration levels of PFASs have been frequently reported in plants near contaminated sites [80, 81, 82, 83, 84]. The predominant PFASs have been PFOA, perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA), and perfluorobutanesulfonic acid (PFBS) in most cases [83, 84, 85]. Table 2 depicts the bioaccumulation of these PFASs in select cereal, fruits and vegetables, some of which are consumed by Africans but not produced locally. It is worth mentioning that the prevalence of these substances in these plants implies exposure to PFASs through the consumption of these crops. More studies are thus required, in this regard, to substantiate this potentiality.

Plant	PFASs (ng/g/dw)
Cereals	PFOA	PFBA	PFPeA	PFHxA	PFBS
Corn	2478.44	1448.59	387.68	116.06	0.29
Maize	0.40	37.37	7.65	13.04	<0.05
Rice	1.73	n/i	n/i	n/i	n/i
Soybean	3966.62	2378.31	992.62	211.80	<0.02
Wheat	809.75	1102.51	495.77	134.69	0.51
Vegetables
Cabbage	1.94	17.85	1.79	0.56	n/i
Carrot	1468.08	2552.74	852.31	196.85	1.10
Cauliflower	86.08	194.10	78.32	32.79	<0.02
Celery	1119.41	1049.61	324.06	94.30	<0.02
Cucumber	2.60	63	0.85	0.32	15
Lettuce	1038.27	2365.18	281.17	72.19	<0.02
Pepper	39.29	946.46	415.86	74.39	<0.02
Pumpkin	15.09	638.13	64.10	11.65	<0.02
Radish	1879.76	1167.52	426.45	103.31	<0.02
Spinach	2.49	6.70	1.79	3.90	0.17
Tomato	1.70	87	1.30	0.56	13
Welsh onion	360.58	270.39	77.79	30.73	0.07
Yam	110	n/i	n/i	n/i	40
Zucchini	3.20	69	3.10	0.28	11
Fruits
Grape	1.60	9.80	n/i	1	n/i
Muskmelon	1	2.90	n/i	n/i	n/i
Peach	1.30	n/i	n/i	n/i	n/i
Pear	1	3.70	n/i	n/i	n/i
Sugarcane	110	n/i	n/i	n/i	n/i
Watermelon	7.90	3.60	n/i	n/i	n/i

Table 2.

Bioaccumulation of PFASs in edible plants.

Short-chain PFASs are shown in bold type. Italic and bold are plants that are likely to be find in selected supermarkets in sub-Saharan Africa, but which are only produced in selected countries of the region (e.g., South Africa and Namibia).

n/i: not indicated.

Adapted from [80].

3. PFASs as a threat to sub-Saharan African medicinal plants

For centuries, medicine plants have played a therapeutic role in the lives of millions of people in developing countries worldwide, and in sub-Saharan African regions, in particular. In addition, it has been reported that, due to their bioactive organic chemical compounds content, also referred to as phytochemicals, these plants have been able to play a defensive role against major chronic ailments in both host-metabolic or genetic dysfunctional and infectious diseases, thus making them beneficial for human and animal health [86, 87]. In sub-Saharan African countries (Figure 1), millions of people depend on medicinal plants for their primary healthcare therapy for obvious reasons such as, these people are inhabitants who live closer to the natural vegetation such as forests, with an estimated 216,634,000 ha of closed forest [12] and savannas_ the later having been reported to be rich in biodiversity with an estimated 71% of vegetation of these ecosystems being medicinal plants, the easy and free access to these plants, as well as the prohibitive cost of orthodox products [87, 88].

Furthermore, the reviewed literature has reported that many different plant species might be used to treat specific ailment(s) in various sub-Saharan African countries, as well as a particular plant being used for the same kind of illness in two or more countries, thus implying the variety and abondance of these plants in the region and the history that these countries previously shared. For example, a recent article reported on antimalarial medicinal plants used in Benin, Burkina Faso, Cameroon, DRC, Ethiopia, Gabon, Ghana, Guinea, Kenya, Mali, Namibia, Nigeria, Uganda, Senegal, South Africa, Rwanda, Togo, Zambia and Zimbabwe (thus representing all the sub-Saharan regions, see Figure 1), with the following plant species used in numerous countries, namely: Azadirachta indica (Benin, Burkina Faso, Ghana, Guinea, Ethiopia, Kenya, Nigeria, Togo, Uganda and Zimbabwe), Nauclea latifolia (Benin, Cameroon, Gabon, Ghana, Guinea, Kenya, Nigeria, Senegal and Togo), Carica papaya (Benin, Ghana, Guinea, Nigeria, Togo, Uganda, Zambia and Zimbabwe), Cassia siamea (Benin, Burkina Faso, Ghana, Guinea, Nigeria, Togo, Zambia and Zimbabwe), Ficus sur Forssk (Burkina Faso, Gabon, Guinea, Kenya, Namibia, Nigeria, Togo and Uganda), Cassia occidentalis L. (Benin, Kenya, Ghana, Namibia, Nigeria, Zambia and Zimbabwe), Jatropha curcas L (Benin, Ghana, Guinea, Ethiopia, Nigeria, Uganda and Zambia), Maytenus sp. nov. A. (Benin, Guinea, Kenya, Nigeria, Senegal, Sudan and Zambia), Tamarindus indica L. (Benin, Ethiopia, Guinea, Kenya, Uganda, Togo and Zambia), Vernonia amygdalina (Benin, Ghana, Kenya, Namibia, Nigeria, Uganda and Zambia), Tithonia diversifolia A. Gray (Burkina Faso, Guinea, Nigeria, Uganda, Rwanda and Zimbabwe), Adansonia digitata L. (Benin, Namibia, Nigeria, Togo and Zambia), Momordica foetida Schumach. (Ethiopia, Ghana, Uganda, Zambia and Zimbabwe), Securidaca longepeduculata Fresen (Namibia, Nigeria, South Africa and Zambia), and Flueggea virosa (Willd.) Voigt (Benin, Kenya, Togo and Uganda), Ximenia americana L. (Guinea, Kenya, South Africa and Zambia), and Zanthoxylum chalybeum Engl (Kenya, Rwanda, Uganda and Zambia) [11]; and for the management of human immunodeficiency virus (HIV) and other sexually transmitted infections (STIs) in the sub-Saharan region, Ximenia americana has been in use (Zambia, Uganda and Kenya), and Azadirachta indica (Uganda and Kenya) [89, 90, 91]. Similarly, numerous studies have highlighted the anti-diabetic potential of several hundreds of sub-Saharan African medicinal plants [4].

Additionally, a valuable review article has highlighted the commercial importance of African medicinal plants, including 13 species (i.e., Catharanthus roseus, Centella asiatica, Coffea arabica, Cissampelos pareira, Cyclopia genistoides, Pausinystalia johimbe, Synsepalum dulcificum, Sclerochiton illicifolius, Strophanthus gratus, Physostigma venenosum, Thaumatococcus daniellii, Voacanga africana and V. thouarsii) which are sources of commercially important chemical constituents, and has, to some extents, represented approximately 10% of commercially developed medicinal plants [13]. The review has further suggested that presently Africa is home to more than 80 beneficial commercial medicinal plant species that are on international markets, and this, in our view confirms the argument that the demand for medicinal plants, including African, is increasing at an alarming rate [7]. Interested readers are encouraged to read this remarkable review article [13] as it depicts a well-informed selection of most popular and important medicinal plants distributed in the different sub-Saharan regions (Figure 1).

Nevertheless, albeit the promising prospects of these plants as alluded to, knowledges on African medicinal plants are still very limited in comparison to other societies, such as the Chinese and Indian; this is so because, unlike in China and India where medicinal plants have extensively been researched and documented, studies of African medicinal plants have not been taken seriously. For example, recent evidence from sub-Saharan-southern African region highlighted 257 plant species from this region that are used traditionally for the treatment of viral respiratory ailments, but only one of these plants has this far been tested for its ability to constrain respiratory viruses by means of its ethnobotanical usage [92]; while of the 555 medicinal plants identified to treat inflammation and pain, from the same region, only few have been relatively screened for their anti-inflammatory properties, which prompted the researchers to recommend that further studies be undertaken in that regard [93]. Substantial, this lack of seriousness in this domain has led to information on African medicinal plants either being unavailable or fragmented and, in the end, incompletely documented [12, 13].

Certainly, the challenges that sub-Saharan African medicinal plants are subjected to are numerous [4]. For instance, it has been indicated that there’s an urgent need to increase the documentation on sub-Saharan African medicinal plants because of their accelerated losses due to anthropogenic activities [12]. For example, the rate of the loss of natural forest cover or deforestation on the African continent is one of the highest globally [7, 12]. Basically, the global deforestation rate stands at 0.6%, but this rate is at 6.5%, 5.0% and 2.1% in sub-Saharan Africa for countries such as the Cote d’Ivoire, Nigeria and the DRC respectively [12, 94]. Additionally, sub-Saharan African medicinal plants are affected by unsustainable harvesting methods [7, 11, 13, 16, 87], fires, wattle expansion or eradication program and grazing [7, 87], coupled with human settlement expansions, including urbanization, as well as inexistent or weak legislations and/or enforcement failure of existing rules and regulations [16].

Moreover, the most recent challenge threatening the prospects of sub-Saharan African medicinal plants, in our opinion, is the contamination of these plants by PFASs. Hence, unlike in the developed world, where the assessment and monitoring of PFASs prevalence in the natural environment are at an advanced stage [4], it is only recently that PFAS studies from the sub-Saharan African region have started emerging [54]. Thus, during the last two decades there has been reports on PFAS from South Africa, Nigeria, Kenya, Ethiopia, Ghana, Burkina Faso and Ivory coast, Tanzania and Uganda [4, 54, 95, 96]. It is worth mentioning that the continent has over 50 countries, which suggests that PFAS studies are still limited on the continent at large. All available evidence has reported higher level concentrations of specific PFASs in analyzed samples, compared to allowed international standards. For example, higher levels of PFOS and PFOA were reported in tap water from Ghana, but lower in tap and bottled water from Burkina Faso [95]. Similarly, PFASs have been reported in wastewater from wastewater treatment plants (WWTPs) and ultimately in several surface water systems in sub-Saharan Africa, including in South Africa, Kenya, Nigeria, Ghana and Ethiopia [54, 95, 96, 97]. Hence, it can be argued that inefficiently treated wastewater represents a risk to plants, including medicinal plants, to which PFAS-contaminated water is or might be applied to. And this is substantiated by the literature that has confirmed that WWTPs are PFAS-contamination hotspots, are considered as the most common point sources of PFASs to surface water [98, 99]. Similarly, available evidence has indicated that the cultivation of medicinal plants at a commercial scale has started emerging from the sub-Saharan African region, with countries such as South Africa, Uganda, Kenya, Tanzania and the DRC having taken the initiatives toward the commercial cultivation of these plants, with high probabilities that surface water (e.g., river water) is used or luckily to be used, like it is the case in South Africa, to irrigate the lands on which medicinal plants are planted with such water, to alleviate the burden of water shortage, for instance. This is a huge potential risk and a threat to medicinal plants.

Furthermore, there are considerable evidence on the prevalence of PFASs in edible plants we have previously alluded to, in the general environment worldwide [80, 81, 82, 83, 84, 85], but the state of these substances, in this regard, in the sub-Saharan African region remains largely unknown and undocumented. However, several countries from this region have been commercially trading with world leading economies (e.g., China and USA) from which PFASs have been reported not only in their natural environments, but also in consumer products that are imported from these world greatest economies by their African commercial patterners [58, 79]; this implies the potential prevalence of PFASs in the general environment of this region as previously alluded by Ssebugere et al. [95] who suggested that the likelihood of PFASs in African environments would certainly be due to the uncontrollable and unregulated importation of PFAS-carrier products from these mass manufacturers to the African countries. Further studies are overdue to substantiate these conjectures, which, if confirmed, represent substantial threats to the general African environment.

Similarly, available data further suggest a greater need for investigations to be conducted on the uptake of these compounds by medical plants. In fact, to our knowledge, there is only a single article that has recently reported on the susceptibility of medical plants to PFASs, with the African marigold (Tagetes erecta L.) as a typical example [100]. The results from the later study have become a wakeup call, as they suggested medicinal plants as possible conduits of PFASs, including PFOA (94.83 ng/g), PFOS (5.03 ng/g) and PFBS (1.44 ng/g), into humans; and thus represent a huge threat to the entire potential medicinal plants industry of the sub-Saharan African region; and the situation is expected to be exacerbated as the continent embarks on developmental trajectories. Therefore, for this region to remain certain that it maintains its remarkable medicinal plants history and keep its population safe, we are of the opinion that studies, in this regard, should be expanded and diversified in the region, in order for a database of these chemicals on medicinal plants to be efficiently established for the whole of the sub-Saharan African region.

4. Conclusions

The use of medicinal plants to combat diseases and illnesses from which humans suffer is not new. It can thus be said that these plants have a long history. In sub-Saharan Africa the history of medicinal plants is remarkable, huge and divers, transmitted by word of mouth from one generation to another using the thousands of dialects present in the region. Owing it to its tropical and subtropical geographical positions, sub-Saharan Africa is said to be home to an enormous biodiversity of medicinal plants, with over fifty thousand plant species, a quarter of which is well known for their curative potentials. Notwithstanding these possibilities, the region still lacks its own drugs on the global markets due to the limited and/or small-scale type of cultivation of medicinal plants still in practice in the region, coupled with several other challenges that medicinal plants are faced with, including not being sufficiently undocumented, deforestation, conservation inefficiencies, overexploitation or overharvesting, etc. Hence, there isn’t any doubt that medicinal plants in sub-Saharan African region are subjected to several encounters. But the most recent of these encounters have been the potential threats from emerging pollutants, i.e., PFASs. These substances are the results of anthropogenic activities unlike their predecessors, the heavy metals, which are naturally find in the environment. To date, there has been over 5000 PFASs manufactured since their dawn in the 1950s. During the manufacturing process that gives life to PFASs, they’re given unique properties that make them wanted by several manufacturers of consumer products. Unfortunately, due to these compounds being heavily applied in industrial processes, they have now been detected in different environmental matrices, including water, soil and plants, as well as in animals. PFASs have been even detected in samples from remote areas, far from the places where they were manufactured, with PFOA and PFOS being predominantly studied, owing it to their health concerns. Regardless of PFASs being extensively researched and documented in different parts of the world, the chemicals have remained undocumented in sub-Saharan Africa, to a large extent. This situation is highly concerning in the context of medicinal plants of this region, because PFASs have been proven to translocate and bioaccumulate in plants, and linked to numerous severe diseases, including cancer and diabetes. This state implies that plants, medicinal plants in this case, being a possible pathway through which humans get exposed to these human-made substances. And with medicinal plants being the first line of defense to combat diseases, illnesses and other daily health needs for millions of low-income people from sub-Saharan Africa, there is cause for serious concern. It is therefore recommended that PFAS studies be expanded and diversified in sub-Saharan Africa. Future studies should also investigate the prevalence of novel or the so called PFAS-substitutes in African environments. The region needs trade-agreements and regulations that make provisions for PFASs from countries with the reputation of manufacturing these chemicals and their alternatives. To valorize its current medicinal plant diversity, the region needs to shift from small to large-scale cultivation of medicinal plants. Routine-based assessment and monitoring of PFASs, their precursors and alternatives in general African environments is also recommended, with an emphasis on the cultivation, harvest or collection, and storing of medicinal plants in areas free of any possible contamination.

Acknowledgments

The authors are grateful for the support and assistance received from: Mr. Masibulele Fubesi, Ms. Espérance Byamungu, as well as the staff from the Department of Civil Engineering and Geomatics and all BioERG members for unwavering encouragements.

Conflict of interest

The authors declare no conflict of interest.

Funding

The authors would like to acknowledge the funding assistance from the Cape Peninsula University of Technology, through the University Research Fund (URF)-Cost Centre R484.