Environmental exposure assessment of lead and cadmium in street vended foods sold in selected locations in Kenya

Abstract The preparation and handling practices, as well as raw materials for street food vending businesses, could be sources of toxic heavy metals in street vended foods (SVFs). The objective of this study was to assess the levels of lead (Pb) and cadmium (Cd) contamination in ready‐to‐eat SVFs sold in selected locations within Thika town, Kenya. A total of 199 samples consisting of cereal‐based foods, sliced fruits, salads, groundnuts, tubers, fresh fruit juices, eggs, smokies, and sausages were randomly collected for analysis. The concentration of Pb and Cd in street vended foods (SVFs) was determined by atomic absorption spectrophotometry. The results indicated that at least one of the food sample types was contaminated with Pb. The level of Pb contamination in SVFs ranged between 0.271 ± 0.070 and 1.891 ± 0.130 mg/kg with groundnuts recording significantly (p < .0001) higher levels (1.891 mg/kg) than all other food samples. Cadmium contamination levels in the SVF samples ranged between 0.001 ± 0.001 and 0.010 ± 0.003 mg/kg. Significantly (p < .0001) high levels of Cd were observed in cereal‐based foods (0.010 mg/kg) and fresh fruit juices (0.008 mg/kg). The Pb concentrations reported in this study are a food safety concern since they exceed the maximum recommended limits set by the Joint Food and Agriculture Organization (FAO)/World Health Organization (WHO) food standards program. There is therefore a need for the establishment and enforcement of policies to govern the street food vending businesses to reduce heavy metal contamination in the SVFs.

the chemical contaminants that may potentially contaminate street vended foods (SVFs). Contamination may occur at any step from the production of raw materials to street food preparation and eventual vending (Proietti et al., 2014). It has been reported that the use of unsuitable cookware, utensils, and food packaging material could release harmful chemicals such as heavy metals into the food (Letuka et al., 2023;Pereira et al., 2022). In addition, the source of raw food products and exposure to environmental contaminants, such as vehicle exhaust emissions and airborne dust particles may cause contamination of food with heavy metals (Ali & Al-Qahtani, 2012).
For instance, the vegetables used by street food vendors may be contaminated with significant levels of heavy metals due to farming on contaminated soils, irrigation with polluted wastewater, as well as industrial and transportation emissions that are released into the environment (Ankar-Brewoo et al., 2020;Fasinu & Orisakwe, 2013;Iriabije & Uwadiae, 2020;Osaili et al., 2016). The growing of vegetables in cities and most industrial places has been reported to yield foodstuff contaminated with heavy metals (Ali & Al-Qahtani, 2012). Njagi et al. (2017) reported that vegetables grown around dumpsites in Nairobi city County in Kenya were heavily contaminated with mercury (Hg) and lead (Pb). Foods of animal origin are also at risk of contamination with heavy metals due to exposure during the feeding of the animals, transportation, and handling practices at the retailing stages (Ankar-Brewoo et al., 2020).
Several researchers have reported the presence of heavy metals including Pb and Hg in SVFs (Badr & Arafa, 2023;Chavez et al., 2014;Ekhator et al., 2017;Ihsan & Edwin, 2018;Mohammed, 2010). In a study carried out in Sudan, Pb was found in various food products including ice cream and peanut butter (Mohammed, 2010). Lead, cadmium (Cd), and Hg were also reported in vegetables, cereals, and fruits sold in Saudi Arabian markets (Ali & Al-Qahtani, 2012).
In Nigeria, Ogunkunle et al. (2014) reported the presence of Pb, Cd, copper, nickel, and cobalt in street vended mangoes, pawpaw, and watermelons while Ekhator et al. (2017) reported the presence of Pb, Hg, antimony, manganese, aluminum, and Cd in SVFs in Nigeria.
In Kenya, research on heavy metals especially RTE food products is limited. Kinyanjui (2009) found Pb levels of up to 0.38 mg/100 g in fruits and vegetables sold in Kisumu, Kenya.
Heavy metals are extremely toxic to human health and are capable of bioaccumulation (Ismail et al., 2017;Khan et al., 2015;Letuka et al., 2023). Continuous ingestion of heavy metals has a damaging effect on human health (Israel et al., 2020;Jaishankar et al., 2014). The adverse effects of heavy metals on human health include structural damage, renal failure, damage to cells and tissues, osteoporosis, lung or even blood cancer, hormone imbalances, gastrointestinal problems, and anemia (Ismail et al., 2017;Letuka et al., 2023). Previous research has also linked heavy metals to the increasing cases of cancer in sub-Saharan Africa (Fasinu & Orisakwe, 2013). Problems caused by these toxic contaminants may not be immediate but accumulation in the body may reach toxic levels causing health issues later in life (Ogu & Akinnibosun, 2020). For SVFs that have minimal regulation, research on heavy metal contamination can inform the making of policies to govern street food quality. However, there is little knowledge of dangerous chemical pollutants such as levels of heavy metals in the SVFs in Kenya. Knowledge regarding concentrations of heavy metals in foods and their dietary intake is very useful in the determination of their risk to human health (Ankar-Brewoo et al., 2020;Khan et al., 2015). Therefore, this study aimed to determine the level of selected heavy metals in SVFs sold in Thika town, Kenya.

| Study site
This study was carried out in six locations including Thika town center, Goingwa, Juakali area, Makongeni, Kiandutu, and Thika Level 5 hospital area in Thika town, Kenya as shown in Figure 1. Heavy metal analysis was carried out at Chuka University, Department of Physical Sciences, Chemistry laboratory.

| Sampling of street vended foods
The sampling of street vended food samples was done from randomly selected street food vendors in each study location. There were 12 ready-to-eat food sample types on offer by street food vendors which were selected for analysis. The food samples were categorized as either food of animal origin (sausages, smokies, and eggs) or foods of plant origin (cereal-based foods, french fries, groundnuts, fresh fruit juices, sliced watermelons, sliced pineapples, salads, arrowroots, and sweet potatoes). Approximately, 200-300 g of each food sample was collected from the randomly selected street food vendors and bagged in a sterile 500 g sample bag. Samples were immediately transferred into a cooler box (4°C) for transportation to the laboratory for analysis.

| Preparation of reagents and standard curves
All reagents used were of analytical grade. Analytical standards for Pb and Cd were procured from Sigma Aldrich Company. The standard analytes for the calibration curve were made by diluting a 1000 mg/L stock solution of the examined element. For each of the elements to be examined, different concentrations were prepared from the stock solution and a calibration curve was plotted using concentration and absorbance data from each set of standards.

| Sample preparation, digestion, and analysis
The presence of Pb and Cd in food samples was determined using atomic absorption spectrophotometer (PG 990 model) as described by Puwastien et al. (2011). Solid samples were dried at 105°C for 24 h and homogenized in a laboratory grinder. Fresh fruit juices, sliced watermelons, sliced pineapples, and salads were blended into a homogenous mixture and used in their liquid form for analysis. Dry ashing was done in a muffle furnace at 525°C for 8 h using 3 g of the dried samples and 10 g of the fresh fruit juices, sliced watermelons, sliced pineapples, and salads. The fresh fruit juices, sliced watermelons, sliced pineapples, and salad samples were first dried in an oven at 105°C until all moisture was removed before charring and ashing.
The ash obtained was dissolved in 1 N Nitric acid solution, filtered using the Whatman filter paper No. 1, and the volume adjusted to 50.0 mL with distilled water. A blank sample was prepared in the same way. The instrument parameters were as follows: wavelengths were 283.3 and 228.8 nm, for Pb and Cd, respectively. In both cases, the lamp current was 2.0 A while the slit was 0.4 nm. The limit of detection (LOD) and the limit of quantification (LOQ) were determined using blank determination as described by Shrivastava and Gupta (2011). The LOD was 0.062 and 0.001 while the LOQ was 0.064 and 0.004 for Pb and Cd, respectively. Measurements were carried out in an air/acetylene flame.

| Data analysis
All analyzed sample results were subjected to statistical analysis using SAS (Version 9.4) software for analysis of variance (ANOVA) and mean separation using Tukey's honestly significant difference (HSD) test at 95% confidence interval.

| Exposure assessment of cadmium in street vended foods
Cadmium (Cd) contamination was only quantified in cereal-based foods, eggs, french fries, fresh fruit juices, sliced watermelons, sausages as well as sweet potato samples (Table 1). Detectable contamination levels in all street vended foods (SVFs) ranged between 0.001 ± 0.001 and 0.010 ± 0.003 mg/kg. Significantly (p < .0001) higher levels of Cd were observed in cereals (0.010 mg/kg) and fresh fruit juices (0.008 mg/kg) as compared with all other samples.
The lowest detectable levels of Cd were observed in french fries although this was not significantly different from the sliced watermelons. Cereal-based foods had the highest number of samples contaminated with Cd which were only found in three study locations including Kiandutu, Ngoigwa and Thika town center (Table 2).
In contrast to this study, Ezeilo et al. (2020) (Bolan et al., 2013). In other studies, the presence of Cd in plant-based foods, such as cereals and fruits has been attributed to Cd impurities in fertilizers and amendments applied to soils (McLaughlin & Singh, 1999). Furthermore, wastewater carrying industrial wastes that may be used to irrigate agricultural lands, the burning of fossil fuels such as coal or oil, and the incineration of municipal waste may be a major source of Cd in the food chain The level of Cd was highest in SVFs sold around the hospital area (0.005 mg/kg) and Thika town center (0.006 mg/kg) as shown in Table 3. As earlier observed, Thika town center also recorded the highest number of samples contaminated with Cd. In addition, at least one foodstuff was contaminated with Cd in all study locations except the Juakali area ( Table 2). The high levels of contamination of foods with Cd in areas around the hospital area and Thika town center could be attributed to the heavy traffic around these places as compared with the Juakali area. Exhaust and non-exhaust vehicle emissions in urban and motorway road dust have been found to contain Cd, among other heavy metals which may contaminate food and the environment (Adamiec et al., 2016;Chenery et al., 2020;Ferretti et al., 1995). Thus, foods that are not covered adequately during preparation or sold along roads that have heavy traffic may be contaminated with Cd to a larger extent as compared with those sold further away. Furthermore, water used for handling food has been reported to be a potential source of Cd in food (Musa et al., 2020).
The presence of Cd in these SVFs poses a health risk to consumers, especially the sick people visiting Thika Level 5 hospital when they consume these foods.
Although the levels reported in this study were below the MRLs established by Codex Alimentarius Commission (2019), the presence of Cd in foods is a major health concern since there is no treatment for Cd poisoning (Ellen & Costa, 2010). Cadmium has been classified by the International Agency for Research on Cancer (IARC) as carcinogenic to humans (Group 1), with sufficient evidence for lung cancer and limited evidence for kidney, liver, and prostate cancer (Joint FAO/WHO Expert Committee on Food Additives, 2011).
Excessive intake and long-term exposure to Cd have also been reported to cause serious illnesses such as itai-itai disease, chemical pneumonitis, and chronic obstructive lung disease (Bolan et al., 2013;Ellen & Costa, 2010;Letuka et al., 2023;Rahimzadeh et al., 2017). Therefore, its presence in SVFs poses a public health concern considering the huge population of people who frequently consume these foods daily.

TA B L E 1
The level of lead and cadmium in specific street vended foods sold in selected locations in Thika town, Kenya.

| Exposure assessment of lead in the street vended foods
At least one of all food sample types was contaminated with Pb (Table 1)     These contaminated discharges are released into the waterways further resulting in the contamination of water and irrigated lands.
In a study carried out to determine the levels of heavy metals in sediments of the Masinga reservoir in Kenya, Nzeve et al. (2018) reported the presence of Pb at levels lower than the WHO set limit.
Thus, street vended foods could have been prepared from contaminated raw materials or water, or exposed to contaminated dust when left uncovered during sale in areas that have heavy traffic. Leaching Lead contamination was highest around the hospital area (0.687 mg/kg) and Ngoigwa (0.935 mg/kg) as shown in Table 3.
Juakali area (0.436 mg/kg) and Makongeni area (0.410 mg/kg) recorded the lowest contamination levels which were significantly (p < .0001) lower than all other study areas. At least one street vended food sample in each food type was reported to contain Pb in all study locations (Table 4) These levels are high and comparable to the contamination levels reported in this study. This shows that Pb is a persistent contaminant in the environment in many places around the world. The high levels of contamination of foods with Pb in areas around the hospital and Ngoigwa could be attributed to the heavy traffic around these places. Exhaust and non-exhaust vehicle emissions in urban and motorway road dust have been found to contain Pb, which may contaminate food and the environment (Adamiec et al., 2016;Ferretti et al., 1995). The low contamination reported in eggs could be because eggs were deshelled before sampling was carried out.
Eggshells have been reported to be an avenue for the excretion of lead in birds (Burger, 1994;Trampel et al., 2003). Thus, contamination with Pb that was possibly in the shell may have been removed when the eggs were deshelled.
Like many other heavy metals, Pb is not biodegradable and thus it does not disappear from the environment over time (FDA, 2020).
Furthermore, Pb builds up in the body over time, and thus, even low-level chronic exposure can be dangerous (FDA, 2020;Letuka et al., 2023;Naranjo et al., 2020). Infants, young children, pregnant women and their fetuses, and individuals with chronic health issues are particularly vulnerable to Pb poisoning (FDA, 2020). According to Dobaradaran et al. (2010), Pb is toxic even at trace levels. Lead may cause renal failure, liver damage, impaired hearing, mental retardation, and shortened gestation in humans (Ankar-Brewoo et al., 2020;Islam et al., 2014). In children, it can cause adverse and permanent neurodevelopmental problems resulting in learning deficits, behavioral problems, and a lower IQ in early childhood (FDA, 2020;Naranjo et al., 2020). Thus, Pb presence in SVFs poses a public health concern considering the huge population of people who consume street vended foods.

| CON CLUS ION
The Pb concentrations reported in this study are a cause for con- food standards program. At least one of all food sample types was contaminated with Pb, with the highest contamination levels being observed in groundnuts. In addition, at least one foodstuff was contaminated with Cd in all study locations except the Juakali area. The presence of Pb and Cd in SVFs poses a public health concern considering the huge population of people who consume these foods.
Furthermore, these heavy metals can bioaccumulate in the body posing health risks later in life. Thus, there is a need for continuous monitoring of SVFs to increase awareness of heavy metal contamination in the food chain. Furthermore, the establishment and enforcement of policies to govern the SVFs businesses aimed at reducing heavy metal contamination are recommended.

ACK N OWLED G M ENTS
We would like to thank the Germany Academic Exchange Service (Deutscher Akademischer Austauschdienst -DAAD) for funding this study.

CO N FLI C T O F I NTE R E S T S TATE M E NT
The authors declare that they do not have any conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data available on request from the authors.