Most research studies do not study the degree of exposure to HAP; they limit themselves to understanding health outcomes due to intensive processes of collecting continuous exposure data (Mulenga and Siziya, 2019; Pritit, 2021). Across Kenya, HAP from BMF use in traditional cookstoves poses the most serious threat to public health, especially to women and children (Gebremeskel and Kabthymer, 2021).

Wood is the most frequently used biomass fuel in Kenya (Ochieng et al., 2013, 2020; Jung and Huxham, 2018), and it is used both as unprocessed wood and charcoal, the latter having a far lower impact on HAP. In Kenya, the use of biomass from wood for energy is common, accounting for 65% of the total primary energy supplies (Welfle et al., 2020). According to an earlier report by the Kenyan government, it is estimated that about up to 80% of Kenyan households depend on firewood for both cooking and heating (GoK, 2014; Ministry of Energy, 2019). This was recently affirmed by Osano et al. (2020). The percentage of households that use solid biomass increases to more than 90% in the rural population (Figure 3; Table 2). Similar figures were recently reported in Homabay County by Kitheka et al. (2019). Notably, the use of biomass fuel is higher in places without electricity connection than in those with grids (Table 3).

Although the usage of solid fuels for cooking has reduced across the globe (Mutahi et al., 2021), inequalities persist, with people in less-developed countries continuing to use and, consequently, suffer the highest pollution (Health Effects Institute, 2019). HAP from solid fuels is hazardous (Pritit, 2021) for those exposed to harmful concentrations for longer durations. It is well-established that pollutants emitted from indoor cooking are extremely hazardous. For example, indoor cooking with solid fuels regularly produces harmful levels of PM, over 10 times the accepted levels (Ndwiga et al., 2014; Pratiti, 2021). In a study over western Kenya, Dida et al. (2022) observed that wood and kerosene were the most used fuel types for cooking and lighting at 97.4% and 96.8%, respectively. As a result, the study established that smoke from the wood was the leading (96.8%) source of HAP.

The level of harm depends on the duration of exposure to these pollutants. The exposure degree can depend on many factors, including proximity to the source of the pollutants, time duration of contact with the pollutants, intensity of the pollutants in the household, and the number of households using BMF (Bruce et al., 2000; Polsky and Ly, 2012). For example, women and children are typically more exposed than men since they are physically present for more hours (Naz, 2017) and during the hours with the highest intensity of pollutants. Women who cook with solid fuel derived from high-intensity episodes experience half of the total exposure when they are close to the fire, especially when starting or stirring the fire (Dasgupta et al., 2004). In Rwanda, Das et al. (2018) observed that children residing in groups of enclosed dwellings, in households that cook indoors, and in households proximate to tree cover, are significantly more likely to be affected by respiratory infections. Studies using personal monitors recorded peak concentrations ranging from 20,000 to 50,000 μg/m³ near the cooking fire (Saksena et al., 1992; Ezzati et al., 2000). Ezzati and Kammen (2001) studied HAP in Kenya by collecting data on the location and activities of all household members and the spatial dispersion of HAP over 200 days for 14–15 h per day. Since one’s exposure to HAP varies throughout the day, the study aimed at developing a continuous exposure–response relationship. The study collected data on specific health outcomes, specifically the development of acute lower respiratory infections (ALRIs). The work demonstrated a log-linear relationship between PM exposure and relative risk of ALRI.

Muindi et al. (2016) assessed the levels and sources of HAP in the urban slums of Nairobi (Korogocho and Viwandani). Different from rural settings, in the urban slums, the study noted that in most of the households, over 50% used kerosene as cooking fuel. According to the study, the mean PM_2.5 levels were high in households under study. It was noted that PM_2.5 levels varied highly, especially in the evenings. The average PM_2.5 levels measured within homes at both sites were 108.9 μg/m³ and 59.3 μg/m³ in Korogocho and Viwandani, respectively. The residents of the two slums under study are exposed to high levels of PM_2.5 in their structures. This situation may be the case for other informal settlements in Kenya and its neighboring countries. In western Kenya, Yip et al. (2017) assessed personal exposures in two indoor settings: improved cooking stoves (ICS) and traditional cooking stoves (TCS). The study reported a reduction in mean 48-h PM_2.5 and CO concentrations in ICS compared to TCS. Similar findings were reported by Dianati et al. (2019) in their study on Korogocho and Viwandani slums in Nairobi. Despite the reduction, the level of concentrations for the two pollutants remained above WHO’s air quality guidelines, calling for further reduction measures.

Disease burden from HAP is a consequence of exposure to extremely toxic pollutants produced by BMF burned in open fires or stoves in the home for cooking or heating. The most harmful of these pollutants include CO and particulate matter. Evidence of exposure from cooking indoors with BMF comes from several studies (Smith, 2000; Dasgupta et al., 2004; Polsky and Ly, 2012; Hanna et al., 2016; Basagaña, 2019), which associate exposure to HAP with death from certain diseases. Strong relations exist between HAP and ALRI (Smith et al., 2000; Ezzati and Kammen, 2001), chronic obstructive pulmonary disease (COPD) (Bruce et al., 2000), and lung cancer in the case of coal smoke (Smith, 1993; Bruce et al., 2000; Fullerton et al., 2008; Epstein et al., 2013; Amegah et al., 2014; Noub et al., 2015; WHO, 2014; Mock et al., 2017; Balmes, 2019).

According to the WHO (2014), based on data from 2012, an estimated 4.3 million people die prematurely annually from illness attributable to the HAP caused by the inefficient use of solid fuels. Among these deaths, 12% are due to pneumonia, 34% from stroke, 26% from ischemic heart disease (coronary artery disease), 22% from COPD, and 6% from lung cancer. The poorest and most vulnerable populations in developing countries are generally the most exposed to HAP from biomass combustion (Mishra, 2004; Agrawal, 2012; Smith et al., 2014). Earlier findings on the association between air pollution and preeclampsia (a pregnancy complication characterized by high blood pressure), mostly conducted in developed countries, however, are limited and have been inconsistent (Rudra et al., 2011; Lee et al., 2013; Pereira et al., 2013).

A study by Smith et al. (2004) established that the disease burden attributed to the use of solid household fuels is dominated by that caused by ALRI in young children, which accounts for 59% of all attributed premature deaths and 78% of DALYs. COPD accounts for all the remaining deaths, with the burden from lung cancer a minor contributor, owing to the concentration of estimated use of coal in two sub-regions only. ALRI in children does not cause many years lost due to disability. However, COPD is responsible for a much larger portion of total disability.

In Kenya, the health impacts of biomass fuel use for cooking and heating are evident too. The Ministry of Energy (2019) noted that the Kenyan Ministry of Health estimated that 21,500 premature deaths occur yearly as a result of air pollution due to cooking. According to an earlier report by Stockholm Environment Institute (2016), it is estimated that acute lower respiratory infections are responsible for the second largest cause of death, accounting for 26% of all deaths reported in Kenyan hospitals.

A research study conducted by Majdan et al. (2015) in rural areas of Kwale County in Kenya assessed the total levels of indoor smoke that can be measured in association with typical local housing design and cooking practices in an ordinary rural Kenyan setting. Data were collected from 170 respondents using the inclined respiratory questionnaire. Findings suggested that biomass fuel–related HAP remains a public health challenge in Kwale. More than half of the respondents reported respiratory sickness once to twice a year, while over a third experienced such incidences three to five times a year. Asthma or COPD cases were reported in about 11 respondents, representing 6% of the total respondents.

In western Kenya, Dida et al. (2022) associated a number of health complications with indoor air pollution emanating from wood fuel and kerosene. According to the study, 92.0% of women and 95.4% of children experienced coughs of varying intensities during the year. Furthermore, women (31.5%) reported wheezing, 98% of which felt fatigued and headache. The study concluded that the use of wood fuel in the region increased coughing, phlegm, wheezing, eye problems, and headaches, especially in women and children.

In a related study, Agarwal et al. (2018) investigated and reported that HAP is associated with altered cardiac function among women in Kenya. In a study in which all interviewees used wood as the primary fuel in a traditional cook stove, CO and PM_2.5 concentrations were found to be high inside their homes. Furthermore, the findings reported a high possibility of western Kenyan inhabitants being at risk for cardiac dysfunction with increased cumulative exposure to household air pollution.

Wafula et al. (1990) assessed the indoor air pollution level and investigated its relationship with acute respiratory infections (ARI) in children below the age of 5 in the Muranga district of central Kenya. Evening peak levels of respirable suspended particles (RSP) from biomass combustion were observed. Such concentrations of RSP are known to cause harmful health effects, especially among pre-school children and women. The findings further showed high concentrations of selected polycyclic hydrocarbons in the particulate material.

Given the impacts associated with HAP, there is a need for implementing intervention measures to combat it. The interventions are alternatives to the traditional practices of household biomass fuel use for lighting, cooking, and heating. Unfortunately, there is lack of evidence on affordable, effective, and culturally acceptable interventions in most low- and middle-income countries, hence limiting their uptake (Woolley et al., 2021).

Many governments, non-governmental organizations, and international institutions have begun to devise strategies for reducing HAP for individuals at the bottom rungs of the energy ladder. One such method has been to subsidize cleaner fuel technologies (Ndwiga et al., 2014). However, other than being costly, this strategy is not very feasible in developing countries due to low levels of disposable income (Abdul Latif Jameel Poverty Action Lab, (2020).

On the other hand, electric stoves are not practical in areas with low electricity connectivity (Lai et al., 2020). Even with subsidies, kerosene and LPG are still expensive for poor households and have the additional difficulty of transportation to rural areas (Jung et al., 2018; Ministry of Energy, 2019). Additionally, these subsidy programs are expensive, especially in an age when most governments are trying to trim already overstretched budgets (GoK, 2015). According to the Ministry of Energy (2019), the distribution of types of LPG devices shows that most people are using 6-kg cylinders instead of costly ones. Dida (2022) has attributed the dominance of biomass fuel in the rural setting of western Kenya to poverty, calling for supporting the impoverished households as a way of increasing the uptake of other forms of energy (Figure 4).