The purpose of healthcare systems is to restore health and save patients’ lives, but sometimes adverse effects on the health of healthcare personnel and communities due to unsanitary methods of disposing of HCW is observed (Arab et al., 2008) . Poorly managed waste can cause long-term and undesirable risks to public health and is a potential source of re-infection, posing a significant threat to the environment. Therefore, the management of HCW requires special attention and should be considered a high priority (Wafula et al., 2019) . The management of HCW is an integral part of national healthcare systems. Safe HCW management practices reflect on HCF service quality and cover all activities related to the generation, segregation, transportation, storage, treatment and disposal of waste (Sahiledengle, 2019) . Adequate management of medical waste in HCFs depends on the waste management team, good administration and organization, careful planning, legal frameworks, adequate funding and the full participation of trained personnel in this process (Awodele et al., 2016) . Healthcare facilities managers are responsible for introducing and ensuring an appropriate waste management system, as well as supervising the compliance with appropriate procedures of all medical staff. Therefore, appropriate education and training systems must be available to all personnel responsible and engaged in both segregation and waste collection processes (Anozie et al., 2017) . In line with WHO guidelines, waste segregation practices should be standardized across the country and included in national regulations for HCW management (Chartier & WHO, 2014) . The key to effective management of HCW is the segregation process at the point of waste generation. Segregation means the separation of various types of waste into different colour-coded containers with corresponding coloured liners at places where they are generated as a first step in HCW management (Akulume & Kiwanuka, 2016) . According to WHO recommendations concerning segregation and collection, a general waste container should be black. Sharp, infectious and pathological waste containers should be marked yellow. Chemical and pharmaceutical waste container should have a brown colour. It is also recommended that almost all waste categories should be collected at least once per day, or when three-quarters of the container is filled. The exceptions to this are pharmaceutical, chemical and radioactive waste, which can be collected on demand (Pandey et al., 2016) .

After segregation, waste is collected and transported outside the hospital or within healthcare facility. The transportation of HCW is usually performed using dedicated trolleys and containers. The trolleys have to be cleaned and disinfected daily. Hazardous and non-hazardous waste has to always be transported separately (Singh et al., 2014) . The waste should be stored in designated rooms and appropriate safety and security measures should be taken. In general, non- hazardous, infectious and sharp, pathological, pharmaceutical, chemical and radiological waste should be stored separately in different places with different characteristics depending on the waste stored (WHO, 2017) .

Infectious waste is a variety of hazardous waste which, due to its pathogenic nature, poses a threat to human health. It should always be assumed that infectious waste may contain various pathogenic microorganisms (Makajic-Nikolic et al., 2016) . Pathogens in infectious waste that is not properly managed can enter the human body through damaged skin (rubbing, puncturing or cutting the skin), inhalation, mucous membranes or by ingestion (Chartier & WHO, 2014) . The greatest risk of transmission of blood-borne pathogens is caused by needle stick and sharp injuries (NSSIs) (Jahangiri et al., 2016) . Healthcare waste can transmit more than 30 dangerous blood-borne pathogens (Yazie et al., 2019) while over 20 other infections can also be transmitted by NSSIs, including syphilis, herpes and malaria. These injuries not only increase the possibility of negative health consequences, but also lead to mental stress, fear, tension and anxiety among healthcare personnel (Ghanei Gheshlagh et al., 2018) . The implementation of safety protocols and compulsory training programs for healthcare professionals can reduce the prevalence of NSSIs and associated infections (Matsubara et al., 2017) . It can be concluded that this type of waste poses a great potential risk to human health (Udofia et al., 2017) .

The most common types of HCW treatments are steam-based treatments (autoclaving, microwave and frictional heat treatments), which are used to disinfect/sterilize highly infectious and sharp waste by subjecting them to moist heat and steam. Steam sterilization is used for sterilization instruments and for sharp and hazardous waste treatments. To reduce the volume of waste, steam sterilization can be combined with mechanical processes, such as mixing, grinding and shredding (WHO, 2017) .

Incineration, which involves waste destruction through burning, removes microbiological hazardous materials, reduces their mass and volume and converts them into ashes. An incinerator that is not properly designed or operated, or is poorly maintained, emits toxic substances into the environment. Incinerators operated at low temperatures generate emissions containing carcinogens such as dioxins and furans, which cause health problems (Njagi et al., 2012) . Incinerators operating at 850˚C - 1100˚C and containing special gas-cleaning equipment can comply with international emission dioxin and furan standards. Technologies used to control emission of dioxins into the environment use activated carbon (AC) adsorption. Before flue gas flows into the dust-collection equipment, AC is injected to adsorb the dioxin and then is blocked by a bag filter (Padmanabhan & Barik, 2019) . Volatile metals, such as mercury, lead, arsenic and cadmium, will damage the immune and neurological systems, as well as the kidneys, brain and lungs. The incineration of high-metal-content materials leads to the spread of toxic metals in the environment (de Titto & Savino, 2019) . Adverse health effects in populations in the vicinity of incinerators, including cancer and reproductive dysfunction have been documented (Domingo et al., 2020) . Bottom ash analyses of incinerated medical waste carried out in Tanzania indicated the hazardous nature of ash resulting from the presence of large amounts of heavy metals (iron, cadmium, lead, copper and manganese) (Saria, 2016) .

The incineration of healthcare wastes not only releases toxic acid gases (CO, CO₂, NO₂, SO₂), dioxides into the environment but also leaves a solid material called ash as residue and includes bottom ash and fly ash which increases the levels of heavy metals, inorganic salts and organic compounds in the environment (Rahman & Singh, 2019) . In an effort to justify a projected management and disposal method, bottom ash is either characterized as dangerous, not dangerous or inert (Gidarakos et al., 2009) . A few epidemiological studies in developed countries have shown that incinerator workers/operators and residents closer to incinerators (<10 km) presented with diseases related to their work environment. These included laryngeal cancer (Michelozza et al., 1998) , gastric cancer (Rapiti et al., 1997) , liver cancers (Elliott et al., 1996) and urinary mutagen (Landrigan et al., 1987) . Other researches also indicate that incinerator operators presented with a significant level of mercury in their hair (Kurttio et al., 1998) , lead and cadmium in blood (Wrbitzky et al., 1995) and hexachlorobenzene in blood/urine (Angerer et al., 1992) . A study that focused on three dumpsites in Kenya found that environmental pollution from uncontrolled solid waste disposal is of major concern and generates chemicals or pollutants that reach their surroundings, such as soil, groundwater resources, and even the ambient air, because of environmentally unacceptable disposal or failure of lining system in the dumpsites (Mugo et al., 2015) .

Developed countries have shifted from incineration of HCW to more environmentally friendly technologies such as microwave, plasma pyrolysis and ionized autoclave, with minimum environmental and fewer health issues, repre- senting less threat to the environment and human health (Zhao et al., 2021) . These alternative HCW treatment technologies are rare in low and middle income countries due to financial implications associated with cost and management (Dinis et al., 2022) . Although current modern incinerators operate within a temperature range of 850˚C - 1200˚C, the entire ecosystem and human health is still affected because of the composition of HCW which contain metals that are non-biodegradable (Wei et al., 2021) . Heavy metals such as Cd, Hg and Pb could cause chronic diseases like cancer and long-term neurological conditions as revealed by recent studies, leading to possible mortality (Tait et al., 2020) . Also, these and other heavy metals can leach through the soil and contaminate drinking water (Mukherjee et al., 2021) , which is then absorbed by plants, animals, and other organisms in the food chain (Feng et al., 2020) . Meanwhile, the accumulated heavy metals cause chronic and acute toxic effects in the various living beings in nature. Composting and vermicomposting (which uses earthworms to consume and recycle the organic waste) are successfully used to break down hospital kitchen waste, as well as other digestible organic and placental waste. Another example of a biological process is the natural decomposition of pathological waste through its burial. Non-hazardous waste should be recycled and regularly collected by the municipalities or transported by the facility to public landfills (WHO, 2017) .

Burying medical waste and depositing them in landfills is also dangerous. HCW is almost always contaminated with pathogens, and leaching toxic heavy metals and chemicals from solid medical waste into the soil occurs in poorly designed dump sites and landfills. The leachate can penetrate the soil and contaminate crops, surface and groundwater resources, posing a risk to human health by consuming water. To control the safety of these methods, hydro-geological conditions must be considered. Landfills should have restricted access, control scavenging, use a soil cover regularly, manage waste discharge, and control surface water and drainage (Udofia et al., 2017) .

When incomplete combustion occurs, persistent organic pollutants (e.g., polychlorinated dibenzo-dioxins, polychlorinated dibenzo-furans, and polychlorinated biphenyls) might be formed (Themba et al., 2023) . Polycyclic aromatic hydrocarbons (PAHs), which are formed during incomplete combustion and are present in bottom ash (Githinji et al., 2024) in the environment, have low biodegradability and are highly carcinogenic to humans (Abdel-Shafy & Mansour, 2016) . A study of bottom ash from healthcare waste incineration in Taiwan region found that the sum of the amounts of PAHs (ΣPAHs) ranges from 162 to 3480 μg·kg⁻¹ (Lee et al., 2002) , and in a study of bottom ash obtained from another healthcare waste incinerator, the sum of the amounts of 11 PAHs was found to be 449.3 μg·kg⁻¹ (Wheatley & Sadhra, 2004) .

Heavy metals in healthcare waste are usually not destroyed by the incineration process but they are concentrated in the bottom ash. Heavy metals in bottom ash come from various sources, such as zinc (Zn) from batteries, nickel (Ni) from stainless steel needles, and the presence of chromium (Cr), which may indicate the existence of plastic in healthcare waste organics (El-Amaireh et al., 2023) . The concentrations of heavy metals vary enormously from study to study, according to (Javied et al., 2008) . This variation is due to two reasons: first, incineration operating parameters (furnace temperature, furnace type, and capacity), second, the nature of healthcare waste fed to the incinerator, and third, the experimental parameters for heavy metals content analysis (sample preparation and analytical method) (El-Amaireh et al., 2023) . In another study, incinerator bottom ash samples were collected from four hospitals in Ghana and analyzed using atomic absorption spectroscopy (AAS) to measure the concentration of heavy metals (Amfo-Otu et al., 2015) . The analysis showed that lead (Pb) and chromium (Cr) were present in large amounts in the samples, with average concentrations of 108.59 and 33.1 mg/kg, respectively. Mercury (Hg) had a minor concentration in the bottom ash samples. High quantities of Pb, Cr, Zn, and Ni had also been observed in another study by Morocco, where the samples were collected from two different incinerators (Bakkali et al., 2013) . On the other hand, iron (Fe) had the highest concentration in studies by (Honest et al., 2020) , with a concentration range of 758 - 3148 mg/kg, and nickel was the lowest; bottom ash samples were collected from six healthcare hospitals and analysed using inductively coupled plasma optical emission spectroscopy (ICP-OES). The same results, where iron was the largest concentration, were detected in a recent study from Iraq; the heavy metal content was determined using AAS; iron concentration in the study ranged between 25.3 and 76.6 μg/g. The presence of iron in the bottom ash is because iron is the major element of medical needles (Selman et al., 2021) .

Some studies have investigated the relationship between particle size and the heavy metal content of healthcare waste incinerator bottom ash. Fine particles were found to be enriched with lead, whereas medium-to-large particles consisted of iron (Racho & Jindal, 2003; Allawzi et al., 2018) . Racho & Jindal (2003) explained these results based on the melting point, where lead has a lower melting point (328˚C), so it would convert totally into small particles of ash. Iron, on the other hand, has a high melting point that may reach 1538˚C; therefore, it cannot melt completely but could break at the incinerator’s temperature. The main advantages of incineration treatment are that it reduces the volume and mass of healthcare waste; 70% of mass and 90% of volume; and has been used as an energy generating equipment (Linh et al., 2020) and destroys pathogens and hazardous organics (El-Amaireh et al., 2023) . Healthcare waste incineration products can be divided into two main parts. The first part is the waste that is released to the external environment, such as fly ash, carbon dioxide, sulphur oxides, and chlorides. The second part is the ash left in the incinerator, called bottom ash. The ash remaining in the incineration chamber constitutes 75% - 90% of the total ash (Jaber et al., 2021) .

Despite of these significant advantages, a serious concern with incinerators is their by-products, including pollutant gases, bottom ash, and fly ash (Linh et al., 2020) . Pollutant gases can include toxic and carcinogenic compounds, which is one of the important reasons for the limitations of loading some waste types such as plastic in the incinerator (Beylot et al., 2018) . Also, incineration residues such as bottom ash and fly ash are known as hazardous waste due to the concentration of compounds such as heavy metals (Bayuseno & Schmahl, 2011) . Every year, million tons of ash are produced in incinerators, and must be properly managed (Gomes et al., 2020) , because bottom ash and fly ash contain a significant concentration of heavy metals, which can be an important pollutant in water, soil, and air (Li et al., 2022) . Currently, there are large amounts of low-standard medical waste incinerators that are being operated by some rural and urban medical institutions in Kenya like in Tanzania, which lack air pollution control devices and without secondary combustion chamber (Manyele et al., 2022) and without burning temperature regulation devices. On the other hand, Kenyan healthcare waste streams, though may be segregated ends up in the incinerator which lead to a large variation in both calorific value and composition. All these factors contribute to uncompleted combustion of healthcare waste. Thus, the composition and distribution of toxic elements in bottom ash from these incinerators may be quite different from that generated from well-equipped large incinerator.

Therefore, the properties of incinerator bottom ash must be extensively investigated before this type of special waste can be reused. The levels of PAHs in five Kenyan hospitals have been studied and reported by (Githinji et al., 2024) . The objective of the current study was to obtain basic information on some heavy metals in healthcare incinerator bottom ash by examining levels of Copper, Zinc, Lead, Cadmium and Nickel in Kenya. This information should be useful for evaluating utilization possibilities. This paper, therefore, presents the results of the investigations that were carried out to determine concentrations of these heavy metals in incinerator bottom ash from the five County hospitals.

Quantities of healthcare waste generated in a given hospital ranges widely depending on different factors such as size of the hospital, number of outpatient, number of other departments and number of beds and cots. Hospital 5 with 218 beds and cots, had the highest volume of healthcare waste generated per day (132.98 - 224.54 kg) while hospital 2 with 97 beds and cots generated the least (58.56 - 98.88 kg) ( Table 8 ). The result of the investigation reveals that, though, there was segregation taking place at the point of generation of infectious and sharps waste by use of the color coding system, all the waste was collected and transported for incineration. All the incinerators assessed in this study across the five hospitals were not equipped with Air Pollution Control (APC) systems which agrees with another study done in Ghana (Dominic, 2016) . The financial implications of cost and management of these alternative HCW treatment technologies discourages their use. As a result, lots of noxious organic and inorganic pollutants that are injurious to human health are released in the flue gas (Adu et al., 2020) .

The HCW generation rates of between 0.61 kg/bed/day to 1.03 kg/bed/day as documented in a study done in Kenyan hospitals were used to calculate the total HCW generated·day⁻¹ (kg) (Nkonge et al., 2014) .

1) This study reveals that the levels of 80% of the metals assessed (Copper, Zinc, Lead, and Cadmium) were above WHO maximum permissible levels while Ni was lower.

2) The incinerator bottom ash studied showed that Cu levels in all the hospitals were above the WHO standards (100 mg/kg). Three hospitals (1, 2 and 5) recorded Zn levels that were above WHO standards. All the hospitals except hospital 3 had Pb levels above WHO standards. All hospitals recorded high levels of Cd than WHO standards except hospital 5 while all the hospitals met Ni WHO standards.

3) The incinerator bottom ash should be classified as hazardous and should not be disposed by landfilling due to the observed high level of concentrations of heavy metals that were above acceptable maximum limits.

4) Incinerators operation temperatures could not be recorded due to lack or defective temperature regulation gauges in all the sampling locations.

1) The anticipated continuous exposure to heavy metals in incinerator bottom ash to healthcare waste workers, waste handlers, incinerator operators and the community may pose direct health risk and should be mitigated.

2) To minimize the impact of incinerator bottom ash to human health and the environment, waste minimization, purchasing strategy and inventory control should be adjusted. Goods with short expiry dates should be rejected.

3) There should be a deliberate training of all actors along HCW management chain and healthcare workers, waste handlers and incinerator operators equipped with PPEs and given the necessary vaccinations.

4) Waste treatment equipment should be revamped to meet the National Environment Management Authority’s (NEMA) requirements.

1) Studies to establish health impacts of the fly ash and gaseous emissions to the incinerator operators, health workers, waste handlers and the surrounding communities where healthcare waste is incinerated should be conducted.

2) Studies to establish the health impacts of incinerator bottom ash to the incinerator operators.

The authors wish to acknowledge the support given by Public Health officers from the Hospitals sampled as well as analysts who helped in the preparation and analysis of samples.