The IEHRA framework has been adopted in the presentation format of the current study results as adopted from Machete (2017). Consequently, the results are presented according to the three IEHRA stages, namely toxicity assessment, exposure assessment and rich characterisation:
The toxicity assessment revealed that BTEX are pollutants released during the burning of firewood. No other potential presence of sources of BTEX such as paints, detergents, upholstery fabrics, furniture wax, adhesives, varnishes, glues, carpets, vinyl floors, cleaning chemicals, air fresheners and cosmetics were found in the kitchens. All the respondents used firewood in open-fire three-stone stoves for cooking, and this practice has the potential to cause adverse health effects through the inhalation of smoke released from combustion. Through experimentation, toxicological assessment revealed that there are several pollutants released during combustion of firewood species. It was revealed that VOCs such as benzene, toluene, ethylbenzene and xylene are released as pollutants from the combustion of the firewood used as fuel in Senwabarwana.
Table 2 presents the detected concentrations of BTEX during the burning of the wood of each tree species. These are the concentrations that the community of Senwabarwana is exposed to during the indoor burning of firewood.
The lowest benzene concentration was detected in motswiri, mokgwa and mohwelere. A high concentration of toluene was emitted by the burning of mushu, followed by moretshe, while low concentrations were obtained from burning motswiri, mokgwa and mohwelere. Ethylbenzene and xylene were emitted from the burning of mushu only and were not detected in the other firewood species. Higher concentrations could be expected during prolonged continuous monitoring as well as in the winter months, when the area becomes prone to pollution accumulation due to climatic conditions.
Few jurisdictions have developed indoor air guidelines. No indoor air guidelines were found for BTEX components in South Africa, and so, relevant guidelines from anywhere in the world could be used; for the purposes of the study, the acute reference exposure level (REL) from California was adopted (OEHHA 2019). Average exposure time for acute RELs is 1 h. REL assumes that toxic effects will not occur until a threshold dose is exceeded (NRC 1994).
According to the World Health Organization (2010), there is no safe level of exposure to benzene. Even though the REL from California was used in the study, it is assumed that benzene has a negative health effect at all levels of exposure. Only mushu exceeded the acute benzene REL from California, while motswiri, moretshe, mokgwa and mohwelere did not exceed the given REL. No toluene was detected because of the burning of mokgwa and mohwelere, while the burning of motswiri, moretshe and mushu yielded concentrations lower than the recommended REL of toluene. No ethylbenzene or xylene was detected associated with the use of motswiri, mokgwa or mohwelere. A lower ethylbenzene concentration than the REL was associated with the use of moretshe, while ethylbenzene associated with the use of mushu exceeded the REL value. Xylene levels lower than the REL value were associated with the use of both mushu and moretshe.
The pollutants listed above can cause various health effects once exposure has occurred. Such health effects vary, depending on the length of time for which an individual has been exposed and the concentrations s/he was subjected to. The respondents in Senwabarwana indicated that they cook once and/or twice a day (morning and/or evening). It is assumed that cooking takes approximately 1 h (USEPA 1996). The kitchens are used mainly for cooking and body warming in winter. It was therefore necessary to assess the kitchen structures as they may influence exposure to firewood smoke during cooking. The characteristics of the kitchens are given in Table 3. Seven per cent of respondents had no kitchens and cooked in the open spaces outside their homes. Although this might reduce personal exposure to pollutants, it affects the environment. Twenty per cent of the homes were made of mud and had thatch roofs, while 73% of the kitchens were made of cement and had corrugated iron sheet roofs.
Dasgupta et al. (2009) studied the effect of building materials on indoor air pollution and found corrugated zinc to contribute the most to healthy air quality, followed by thatch, mud and brick. No chimneys were observed in any of the houses. The kitchens had doors (93%), while 7% had no windows. In some homes (17%), there were small openings in the walls, instead of windows. Some kitchens had two windows (65%) while others had only one window (17%). The national building regulation of South Africa requires the minimum ventilation of a room to be 5% of its floor area. However, none of the kitchens in the study area complied with this requirement due to poor house design. The structural design of kitchens in the study area is characterised by a limited number of windows, the size of which is usually small, thereby limiting the exchange of indoor air with outdoor air.
It was observed that respondents opened windows and doors when cooking to allow ventilation. However, pollutants cannot disperse properly due to poor kitchen structures putting residents at risk (Mukkannawar et al. 2014).
Reported health effects
Short-term exposure to high levels of BTEX can cause acute symptoms such as headache, eye, nose and throat irritation, dizziness, nausea and vomiting and the exacerbation of asthma symptoms (Danish EPA 2016; Pokhrel et al. 2010; Nielsen et al. 2008; Han and Naeher 2006). Long-term exposure to high levels of VOCs can increase the risk of liver damage, kidney damage, cancer and central nervous system damage (ATSDR 2007; IARC 2004).
Figure 1 presents common health effects associated with the use of firewood reported in Senwabarwana.
Majority (46%) of the respondents indicated to have experienced headache when exposed to firewood smoke. Thirty-three percent (33%) of the respondents experienced eye problems which includes sore, red and teary eyes. Relying of self-reported sickness may render the results of the study unreliable; however, literature confirms that exposure to VOCs can irritate the eyes since VOCs are irritants to the eyes and respiratory tract (Pokhrel et al. 2010; IARC 2004). These results are also supported by the results of a case-control study of indoor cooking smoke exposure and cataract prevalence in Nepal and India which found that the use of solid fuel in unfuelled indoor stoves is associated with an increased risk of eye problems (Pokhrel et al. 2005). Twelve per cent of respondents self-reported asthma while 3% self-reported incidents of cancer. Only 1% of the respondents reported pneumonia while 9% reported heart problems and 4% reported incidents of stroke. These results are used to confirm the presence of VOCs in the firewood smoke as indicated in Table 2.
Link between health effects and kitchen characteristics
Figure 2 presents the relationship between health effects and kitchen structures. This is a test of statistical significance to compare the frequency of reported health effects by the absence or presence of chimneys, doors and windows, by roof and wall material. It is observed that respondents whose homes had no roof reported fewer health effects, followed by those whose roof is made of thatch while those with corrugated roofs reported the highest health effects.
All the respondents had no chimneys and therefore reported the highest health effects. Those whose kitchens are made of cement reported the highest health effects compared to those whose kitchens are made of mud walls followed by those whose kitchen walls are made of corrugated zinc sheets. Those whose kitchens had windows reported higher health effects compared to those who use kitchens without windows. This might be due to the fact those without windows mostly has no roof while those with windows are not built according to the national standard making it difficult for outdoor air to dilute the indoor air thereby increasing pollutant level inside the kitchen (Mukkannawar et al. 2014).
This step combines the information from the two previous steps to provide an indication of the nature and expected frequency of adverse health effects in exposed populations. The fundamental assumption of the sampling strategy consists of the fact that measured concentrations represent maximum concentrations to which all individuals could be exposed in the kitchen (USEPA 2014). If this assumption is true, then the risk of developing health effects due to the presence of the studied volatile organic compounds can be assessed as negligible. This holds for all BTEX (concentrations remain below the risk levels). Benzene poses a health risk in households where mushu and moretshe are used. There is, however, no health risk associated with the use of mokgwa, mohwelere and motswiri. Similarly, toluene poses a health risk when mushu and moretshe are used, while there is no health risk associated with the use of mokgwa, mohwelere and motswiri. Ethylbenzene poses a health risk when mushu is used as firewood. Xylene has no health risk associated with the use of all five-tree species.
The type of kitchen structures coupled with the use of firewood however seems to affect indoor air quality and householders’ health. Even though the pollutant’s level is lower than the guideline values, it should be noted that these householders are exposed to pollutants every day.
A limitation of the study is that it was conducted for a limited period; therefore, the estimation of risks over longer periods of exposure could not be assessed.