Health impact assessment of volatile organic compounds (VOCs) emission from the combustion of agricultural wastes

Abstract Emissions of components of VOCs from combustion of agro-waste materials were quantified. Agro-waste materials comprising corn cobs, corn husks, bean chaff and rice husks were burnt at a temperature of 400°C in a muffle furnace, and the emissions were quantified according to standard procedures. Health risk associated with exposure to the emissions, based on carcinogenicity and non-carcinogenicity, was evaluated using excess lifetime carcinogenic risk (ECLR) and hazard quotient (HQ), respectively. The results showed identification of 11 compounds comprising undecane, benzene, bromoform, ethylbenzene, styrene, toluene, p-xylene, 1,3-dimethylbenzene, o-xylene and phenol. The concentrations of the identified compounds ranged from 0.01 to 7.35 µg/m3, with chlorobenzene (emission from corn cobs) having the highest concentration and 1,3-dimethylbenzene having the lowest concentration (across all samples). The results also showed that ELCR and HQ values were below the recommended levels. The result concluded that although short-term exposures to the emission from combustion are not likely to cause health issues, prolonged exposure should be avoided as this could complicate issues for individuals with existing health challenges.


PUBLIC INTEREST STATEMENT
Agricultural activities has increase over the years due to population growth. In other to meet the food demand of the populace the generation of Agricultural waste cannot be ruled out. This study investigate the Impact of VOC's emissions from the burning of Agricultural waste as a waste management method to the ambient air quality.

Introduction
In recent years, the rate of generation of wastes have grown tremendously; and by 2025, the rates are likely to have soared higher significantly to 2.2 billion per year (Cognut, 2016). This continuous increase has created a growing apprehension about disposal and management of the wastes in developing nations, especially in the African continent. In these regions, pollution of environment from municipal, industrial and agricultural activities continues to pose threats to both environment and human health, as heaps of wastes are either carelessly and indifferently dumped in the environment or indiscriminately burnt in an open space, thereby releasing gaseous and particulate emissions into the atmosphere. Agricultural practice has been identified as one of the anthropogenic activities that can cause significant environmental pollution (Fakinle et al., 2021a). For instance, in most developing nations of the world, combustion is a widely used method of removing crop residues and post-harvest agro-waste materials. The method is common, following the harvest of maize, rice, wheat and other arable crops. The practice is considered as a means of disposal of heaps of agricultural wastes in readiness for another farming season. Apart from this, combustion is also a common method of preparing the land for cultivation, especially after a period of fallowing, during which both living and dead vegetations are burnt in order to clear the land as well as to release nutrients to the farmland for the next crop cycle. Agricultural waste burning is commonly practiced not only in Nigeria but also in developing and developed countries. China, India, Thailand, the United States and Philippines are at the forefront in terms of combustion of agricultural wastes, followed by Indonesia, Brazil and Russia (Cassou, 2018;Gadde et al., 2009). African countries are not left out in the menace of agro-waste burning as they represent regions with the most intensive rate of combustion of agricultural waste per hectare of harvested farmland (Cassou, 2018).
The rapid urbanization of major cities in Nigeria and the attitude towards waste disposal have led to serious environment degradation, both at the farmer's municipality and at city levels. Nigerian is an agrarian country, and the recent government's interest in the development of agricultural sector for diversification of economy is expected to result in the generation of greater proportion of agricultural wastes both in the city and suburb regions of the country and thus will require proactive management strategy. In Nigeria alone, over 52 million metric tons of agriculture wastes accumulate in the city and only 30% are collected by the City Council or Waste Contractors leaving the rest to rot, litter and pollute the environment (Owanaba, 2015). In addition, the increasing demand for food in the country has also led to a sharp increase in food production through increased agricultural activities which, in turn, has resulted into production of larger quantity of agricultural wastes.
The increasing multitude of agricultural activities increases the amount of agro-products and this has led to an overall increase in environmental pollution (Bhuvaneshwari et al., 2019) emanating from abandonment or combustion of such wastes. Combustion of agricultural wastes consists of using an integrated furnace with a heat dissipation structure for combusting solid fuel (Wang et al., 2019). Although, burning could be an efficient method to reduce vegetative debris from an agricultural operation and disease control (Lal, 2008), its negative impacts range from the degradation of air quality by smoke particles to the loss of organic materials needed for soil enhancement (Chen et al., 2917;Bhuvaneshwari et al., 2019). Combustion of agro-waste materials may release significant amount of criteria air pollutants into the ambient air, during which the natural quality of the atmosphere is changed, bringing direct or indirect adverse effects on both human health and the environment.
Of interest is the evaluation of VOCs emitted from the combustion of agricultural waste, especially considering the effect of this class of pollutants on the environments. VOCs have a property of being converted into vapour or gas without any chemical change. They are highly reactive and can react with other gases to form other air pollutants after they are in the air. VOCs are emitted from natural sources like forest fires and the transformation of biogenic pre-cursors, nevertheless, human activities have become important sources of toxic VOC emissions into the atmosphere. VOCs have been considered, not only as being toxic but have been reported to play active role in the formation of photochemical smog . VOCs also contribute significantly to ground-level ozone and photo-oxidant production (Mugica-Álvarez et al., 2018). Among the VOCs released into the ambient air, benzene, ethyl benzene, toluene and xylene are of special interest due to the role they play in tropospheric chemistry and their negative health effects on humans. These VOCs in the atmosphere can originate from different sources such as residential heating, biogenic sources, automobiles as well as industrial heating. Exposure to benzene, toluene, ethyl benzene and xylene (BTEX) may results into various health effects such as conjunctival irritation, nose and throat discomfort, headache, allergic skin reaction, dyspnea, declines in serum cholinesterase levels, nausea, emesis, epistaxis, fatigue and dizziness (Cai et al., 2022). The extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed. For instance, the health effects of exposure to these compounds becomes more pronounced on individual with existing health challenges. It is against this background that, among all gaseous pollutants, BTEX are considered as a group of hazardous pollutants which continues to attract global scientific research (Baberi et al., 2022).
Some previous studies on VOCs explored on the analysis of VOCs composition from agricultural food crops using some innovative technologies other than the conventional use of Gas chromatography-mass spectrometry (GC-MS), while others investigated emission of the pollutants from industrial facilities. For instance, Rasekh et al. (2022) presented a non-destructive sorting technique for determining the quality, originality and origin of peppers based on the level of VOCs released. According to the study, VOCs emitted by hot peppers significantly differ from the one released by sweet peppers and thus, the property could be harnessed to develop non-destructive machine. In another study by Dong et al. (2015), VOCs' compositions as well as bioactive components of seven coffee samples were characterized and a total of 77 volatiles were identified and hydrocarbons, acids, aldehydes, and alcohols accounted for more than 90% of the total volatile compounds. The use of opto-electronic nose coupled to a silicon micro pre-concentrator device for the detection of volatile aroma in flavoured waters was also presented by Slimani et al. (2020). Few other documented reports have investigated atmospheric VOCs emitted from industrial and transport activities (Fakinle et al., 2021b;Hu et al., 2018); from fuel evaporation  and from plants as a response to environmental changes such as light, temperature and flooding (Yadav & Pandey, 2018). The results of the investigative study of emissions of intermediate-volatility and semi-volatile organic compounds from domestic fuels used in Delhi, India, by Stewart et al. (2021) showed that cow dung cake and municipal solid waste burning are likely to have significant PAH sources. Amnuaylojaroen et al. (2019) modeled the effects of VOCs biomass burning emission on ozone pollution in upper Southeast Asia and concluded that biomass burning plays a significant role in elevating the production of ozone in the region.
On the emission from the combustion of agricultural wastes, apart from Gadde et al. (2009) and Sirithian et al. (2017) who reported emission of VOCs from combustion of rice husk and maize residues, respectively, majority of the previously documented research on agricultural waste burning emission have focused on emission of other categories of pollutants (; Jain et al., 2014;). Literature search, therefore, revealed that quantification of the emission of VOCs from the combustion of agricultural wastes has not been extensively documented.
In this study, emission of VOCs from the combustion of agricultural wastes as well as health risk assessment of BTEX are investigated in order to determine the contributions of agricultural activities to ambient pollution load and the need to address the issue associated with the air pollution. The health implication of exposure to BTEX from the emission emanating from the combustion of these categories of wastes was evaluated based on carcinogenic and noncarcinogenic risks. For this study, rice husks, corn hubs, corn husks and bean chaff obtained from Landmark University Teaching and Research Farm were used. The choice of these wastes was based on their production in large quantities and subsequent combustion after each farming season. During the combustion at 400°C in a muffle furnace, the emissions were collected in a tube loaded with activated carbon adsorbent material, and then characterized using standard methods. The analysis was done using gas chromatography-mass spectrometry (GC-MS) coupled with flame ionization detector (FID). Nigeria, being a country that relies on agriculture for revenue generation (after oil) and food security, continuous growth of agricultural sector is projected as population keeps growing and consequently, continuous increase in agricultural wastes (especially, wastes from agricultural staple crops) is inevitable. This study will therefore provide information on the contribution of agricultural waste burning to the pollution loading of the environment and the impact of such on ambient air quality. It will also help policymakers to proactively device means of managing agricultural waste for environmental sustainability.

Preparation of samples
The agricultural wastes (corn hubs, corn husks, bean chaff and rice husks) used for this study were obtained from the Teaching and Research Farm of Landmark University, Omu-aran, Kwara state, Nigeria. The farm generates large quantum of these wastes and are subsequently burnt off after each farming season in preparation for a new planting period. The selected wastes were dried to remove moisture content before being crushed using mortar and pestle to increase their surface area in order to achieve fast and complete combustion. The weight of the wastes was then determined on analytical weighing balance. The samples were classified into five and coded as shown in Table 1.

Experimental set-up and procedures
A furnace (Okhard Machine Ltd, 18,116), incorporated with a round necked-thistle tube was used for the extraction of the pollutants.
The neck of the thistle was filled with activated carbon (parked in a sizeable piece of muslin cloth as a filter medium) and then tightly thrust into the furnace flue vent for the trapping of emission from the combustion. Escape of the emission from the thistle tube was prevented by making the mouth of the airtight thistle tube . Prior to sampling, 4 g of activated carbon was baked at 300°C for 2 hours to remove traces of organic matter (Bazan et al., 2016;Mukherjee et al., 2014;Song et al., 2014). The baked activated carbon was then fed on top of a piece of muslin cloth previously placed in the thistle tube and the whole arrangement was then sealed with the aid of a glass rodrubber cork attachment. One hundred grams of each sample was placed in the combustion chamber of the furnace and was ignited by the in-built ignition source of the furnace at a thermostat-controlled temperature. For this study, the samples were burnt at 400°C. Heat was absorbed from the combustion process by the heat exchanger within the furnace and the emission from the combustion was then collected on the activated carbon parked in muslin cloth within the thistle tube at 5 minutes. The choice of muslin cloth was based on its high particle retention capacity (Fakinle et al., 2022). The pollutants-rich activated carbon was immediately transferred into a plastic container and refrigerated at −10°C before further analysis.

Extraction of the pollutants
The procedure for the extraction of benzene, toluene, ethylbenzene and xylene was according to standard method described by NIOSH (1996) and subsequently used by other researchers (Baberi et al., 2022;Habeebullah & Hassanien, 2007;Khoder, 2007). Prior to the analysis, samples were removed from the freezer where they were kept. The pollutant-rich activated carbon was transferred into vials containing 2 ml of distilled carbon disulphide (CS 2 ). The vials containing distilled CS 2 as well as the activated carbon were gently shaken by mechanical shaker for 25 minutes and thereafter left undisturbed for about 1 hour in order to obtain a clear sample solution. Two microlitres of the final sample solution was withdrawn from the vials and injected into gas chromatography (Varian Co, 4000) equipped with flame ionization detector. The capillary column of 30.0 m × 320 µm × 0.00 µm was used with nitrogen as a carrier gas and oven temperature programming from 50°C (held for 5 minutes) to 200°C at 15°C/min to 210°C at 2°C/min for 10 min (Hazrati et al., 2016). External calibration was carried out using BTEX standards. From the chromatogram, the retention times of the standards were used for the identification and quantization of the individual BTEX. All other solvents used were of high purity analytical grade. Statistical Package for Social Science (SPSS) program 17.0 was used for statistical analysis of the results.

Assessment of health effect of BTEX
The health risk of exposure to BTEX was assessed by estimating the carcinogenicity and noncarcinogenicity risks associated with exposure to these pollutants. For carcinogenicity risk, excess lifetime cancer risk (ELCR) was calculated by multiplying Chronic daily intake (CDI) with cancer slope factor (CSF) as presented in equation 1 (Odekanle et al., 2020., Fakinle et al., 2022 ELCR  Table 2.

VOC Quantification and characterization
The VOC analysis carried out on the emissions emanating from the combustions of the sample enabled identification of 11 compounds as presented in Table 3. In the five samples, the identification of the compounds using the data from the chromatographic analysis was achieved by NIST recognition software. The compounds identified were undecane, benzene, toluene, ethylbenzene, styrene, bromoform, p-xylene, benzene, 13-dimethylbenzene, o-xylene and phenol. The concentrations of the identified compounds ranged from 0.01 to 7.35 µg/m 3 , with chlorobenzene (emission from corn cobs) having the highest concentration and 1,3-dimethylbenzene having the lowest concentration (across all samples). Bromoform was not detected in corn cobs (sample AQ1).
Significant difference in the concentrations of the compounds within the samples was noted (P < 0.05). Varying VOCs compositions within the samples are thought to be as a result of the difference in the agricultural waste materials. There was no statistically significant difference in the concentrations of the same pollutant in all the samples (P > 0.05). Although, no statistically significant difference was found between the concentrations of o-xylene and p-xylene within all the samples (P > 0.05), the presence of o-xylene was more pronounced in all samples than p-xylene (except in corn husk).
The mean concentrations of VOCs are presented in Figure 1. Phenol constituted about 26% of the VOC components emitted from the combustion of the agricultural residue, followed by chlorobenzene (24%), while ethyl benzene and 1,3-dimethylbenzene have the lowest mean concentrations.
Phenol has been documented as having a several toxic effects ranging from carcinogenic to kidney and liver damage (Atila, 2019; Although the values obtained in this study were lower than some of the previously reported studies (Jain et al., 2014;Pandey & Sahu, 2014;Jain et al., 2014;Yuan et al., 2017), the potential of agricultural waste burning to increase atmospheric volatile organic compounds established in this study was found to be in agreement with previously documented studies (Gadde et al., 2009). A similar study by Paris et al. (2019) identified more varieties of volatile organic compounds from the combustion of rice than what was obtained found in this study (although the concentration of each compound was not reported). Apart from the fact that higher concentrations of VOCs were reported by Paris et al. (2022), the study also revealed that biomass moisture content can generally increase the concentrations of all the classes of VOCs which in turn can adversely affect the quality of our environmental air. Gancarz et al. (2022)    characterized about 20 different volatile organic compounds (as against 11 identified in this study) from coffee bean all with varying percentage composition and the analysis revealed dominance of azines in the aroma profile. All these documented reports supported the fact that combustion of biomass can generate considerable amounts of VOCs. The difference in VOCs concentrations reported in this study and those from previous research could be attributed to varying experimental set-up and difference in the biomass or agricultural waste samples. Also, though the concentrations of the pollutants obtained in this study are lower than the recommended limits, the addition of these proportions to the existing amount in the ambient air could aggravate health conditions of individuals with existing health challenges. For instance, benzene specifically stays longer in the atmosphere than any other VOC member; with atmospheric lifetime of about 9.4 days, it can also travel longer distance than any other aromatic compound where it can be inhaled (Fakinle et al., 2021b) because it is not easily broken down to other compounds except in the presence of hydroxyl radicals. In fact, nitrate radicals, which is present in the atmosphere, contribute to reduce the extent of benzene degradation.

Health risk assessment
The health risk assessment of BTEX was evaluated by estimating both carcinogenic and noncarcinogenic risks of the pollutants using ELCR and HQ, respectively, and the results are presented  in Table 4. It was noticed that the ELCR values obtained for benzene, chlorobenzene ethylbenzene and phenol ranged from 7.0 × 10 −9 to 5.23 × 10 −6 for all samples, with highest cancer risk found in the exposure to chlorobenzene emission emanating from the combustion of corn cobs (based on highest ELCR value for chlorobenzene), while exposure to air emission from the combustion of bean chaff can pose the least cancer risk. Inhalation is the primary route of exposure to benzene for general and occupational populace, though other routes could be oral and dermal. Benzene can be found in water, air and soil as a result of combustion of coal, oil and any other fuel. Exhaust and leakages can also release benzene into the air. Values obtained in this study are lower that the range of values of 10 −4 to 10 −6 recorded by Baberi et al. (2022). Similar other studies have also recorded higher values of ELCR for BTEX (Dehghani et al., 2019;Hazrati et al., 2016).
The difference in the values could be associated with the different sampling environments and the sources of emissions. While previous studies focused on BTEX in the indoor air of residential and commercial building, the sources of which could have been numerous, this study focused on the BTEX emission emanating from the combustion of agricultural wastes into the ambient air. The ELCR values obtained in this study were below the standard limits set by the World Health Organization and Environmental Protection Agency (Dehghani et al., 2019). Hematopoietic system, nervous system and immune system are some of the primary target organs for acute exposure to benzene (ATSDR-Agency for Toxic Substances and Disease Registry, 2007). Other health effects of exposure to benzene include leukemia, drowsiness, headaches and nausea (Fakinle et al., 2021b). World Health Organization has also reported that a lifetime exposure to 1 µg/m 3 of benzene leads to about six cases of leukemia per million inhabitants (World Health Organization (WHO), 1999). On the other hand, non-carcinogenic risks were estimated based on exposure to toluene and xylene. It was observed that the non-carcinogenic risks associated with exposure to toluene and xylene as calculated using hazard quotient values are not likely to cause health issues. The HQ values of 1.0 is an indication that exposure to the pollutants of interest are likely to cause health challenges, whereas HQ below 1.0 suggests no concern about non-carcinogenicity risk when exposed to such pollutants (Gilbert et al., 2019;Odekanle et al., 2020). It is important to note that toluene can enter the body through inhalation, ingestion or body contact. Exposure to this gaseous pollutant can lead to central nervous system disorder which, if not properly managed, can result into fatigue, dizziness, irritation of the eyes and general respiratory issues (Baberi et al., 2022). The results obtained for xylene in this study are lower than the recommended permissible limits. Environmental Management Authorities in most countries have recommended a permissible limit of 100 ppm in the working environment (Rajana et al., 2019). Although most of the xylene that penetrates into the body leaves within 24 hours after penetration, long-term exposure should be avoided because accumulation of considerable amount may occur on a prolonged exposure, which may result into health hazard such as dizziness, nose and eye irritation, severe lung congestion and even death (Rajan & Malathi, 2014;Rajan et al., 2019). Generally, the concentrations of all the components of VOCs recorded in this study are far lower than the permissible limits recommended by World, health Organization, Occupational Safety and Health Administration (OSHA) and the United States Environmental Protection Agency (ATSDR-Agency for Toxic Substances and Disease Registry, 1990Registry, , 1995Registry, , 2007Registry, , 2017USEPA, 1999;WHO, 2000). Notwithstanding, prolonged exposure, especially by individuals with existing medical conditions, should be avoided

Study limitation
The focus of this study was to quantify VOC emissions released from the combustion of agricultural wastes and to assess health risk associated with the exposure to such emissions. Some limitations in terms of research process due to realistic constraints were noticed. The study was based on laboratory experiments, and thus, the concentrations recorded in the study may vary (lower) than what would obtainable, if the wastes were burnt on the field. Therefore, the amount of these concentrations in the atmosphere, using passive sampler is expected to be higher. Furthermore, combustion temperature of the sample was 400°C, in principle, open burning may occur at higher temperature which may also result in emission of more variety of VOCs. Also, four major agricultural wastes have been selected for this study based on some criteria, the real burning process involves accumulation of several agricultural wastes as heaps, which could result in higher levels and more varieties of VOCs than what this study has reported.

Conclusion
In this study, based on laboratory experiment, VOC emissions from the combustion of some agricultural wastes were quantified in order to assess the potential contribution of agricultural wastes burning on pollution load of atmospheric air. Health risks associated with the exposure to the emissions were also evaluated. The result indicated emissions of 11 volatile organic compounds with different concentrations. Although, the values of VOCs obtained in this study were lower than the results from other studies, it has been shown that agricultural waste burning emits considerable quantity of VOCs which has potential to negatively impact ambient air quality. The study therefore concluded that, although both ELCR and HQ values are below the recommended levels, prolonged exposure to emission should be avoided as this could complicate issues for individuals with existing health challenges. For environmental sustainability, therefore, more environmentally friendly approach of managing this category of waste, rather than burning, is advocated.