Expo-Operator Exposure to Heavy Metals from Wood Pellet Ash – Risk Assessment

Ash is a by-product of wood biomass combustion that must be removed daily from stoves or fireplaces. Therefore, operators or owners are exposed to the potential impact of ash. The goal of this study was to determine whether heavy metal present in wood pellet ash posed a health concern to stove operators/owners. The risk assessment procedure was carried out in several steps, including exposure evaluation, toxicity evaluation, and risk categorisation. The hazard coefficient (HQ) and non-carcino genic hazard index (HI) were calculated for Cd, Cr, Cu, Ni, Pb, and Zn. HQ had the highest value for the ingestion pathway (3.62 ∙ 10 −6 ), and the value for non-carcinogenic HI was 3.70 ∙ 10 −6 . The value HI < 1 suggests that there is no risk to operator health related to heavy metals in analysed wood pellets ash. The carcinogenic risk (CR) was calculated for Ni, Pb, Cr, and Cd, and the values were within the permitted limits. The risk assessment based on HI and CR indicators proved that there was no significant health concern regarding exposure to the analysed ashes.


Introduction
Due to rising natural gas and oil prices, as well as increased environmental awareness, wood is becoming more popular as a source of energy, particularly for thermal energy. In recent years, special emphasis has been placed on the use of wood biomass as an energy source. 1 Advantages of renewable energy sources are widely used, and contribute to environmental protection due to reduced harmful emissions from combustion. 2 Biomass from the industry such as wood waste and residues has been an unused potential in the past. However, in recent times, wood waste and residues are used for the production of especially current forms of wood biomass and pellets. 3 The waste generated by the utilisation of wood biomass is predominantly ash. Ashes are microparticles of material generated when solid fuels are burned. The type of solid fuel (wood, coal, or other high-energy substances) burned and the boilers used in the process determine the composition of the ash.
Our findings show that the ash obtained by burning wood pellets from the Bosnia and Herzegovina (B&H) market contains heavy metals. 4 Therefore, ash engenders significant environmental and human health problems due to enhanced contents of heavy metals. Because proper maintenance of pellet stoves or fireplaces demands the removal of ashes on a daily basis, we can predict that operators (usually the owners) are most exposed to dust and compounds emitted in the interior air from the ashes.
A health risk assessment is a method for estimating the im-pact of specific hazardous compounds on human health or the environment. The risk of heavy metals was assessed in different studies for different types of ash like ash from wood pellets, 4,5 solid fuels mixed with municipal waste, 6 and coal. 7 Therefore, the aim of this study was to assess the potential health effects of heavy metals for operators based on heavy metal levels in wood pellet ash. We calculated the hazard index (HI), total carcinogenic risk (TCR), hazard quotient (HQ), and carcinogenic risk (CR) for heavy metals found in wood pellet ash to determine the corresponding operator's health impacts.

Materials and methods
The model used in this work to estimate operator exposure to heavy metals in wood pellet ash was based on models recommended by the US Environmental Protection Agency. [8][9][10] Health risk assessment for operators was performed for ten samples of ash obtained by combustion of wood pellets ( Table 1). Samples of wood pellets for the determination of heavy metals were prepared by wet digestion with HNO 3 (65 %). Atomic absorption spectrometry (Varian AA240FS) was utilised for Pb and Zn, whereas a graphite furnace (Varian AA240Z) was employed for Cd, Co, Cr, Cu, and Ni. 4 sure by inhalation (inh) was calculated using Eq. (2), while Eq. (3) was used to calculate dermal-route exposure (der).
( ) Reference values of health risk assessment parameters are shown in Table 2.
C UCL (exposure point upper confidence limit -UCL) is an evaluation of the reasonable maximum exposure which is the upper limit of the 95 % confidence interval for the mean. [11][12][13] Eq. (4) was used to determine the C UCL : 9,13 where X (mg kg −1 ) is the mean concentration of each heavy metal, Z α is (1−α) th quantile of the standard normal distribution for the 95 % confidence level (1.645), β is the skewness, n is the number of samples, and STD is the standard deviation.
The hazard coefficient (HQ) was used to calculate the possible non-carcinogenic risk for each heavy metal. Equation (5) was used to calculate hazard coefficient: where BAF is the ratio of metal content that is bioavailable to the total content in the analysed samples, 5 and RfD O is the reference dose of a specific heavy metal (mg kg −1 day −1 ).
The potential non-carcinogens risk to the operators/owners through intake of heavy metals was determined using hazard index (HI) given by Eq. (6). 1

Carcinogenic risk assessment
The carcinogenic risk (CR) can be used to calculate the probable cancer risks associated with exposure to a specific amount of heavy metal in ashes of wood pellets. The CR is defined as the cumulative risk of acquiring any type of cancer throughout a lifetime as a result of the operator's exposure to a carcinogenic hazard over ashes of wood pellets. For calculating the lifetime cancer risk, the following equation was used: where SLF is slope factor, values are from Regional Screening Levels. 5,14 The total cancer risk (TCR) associated with exposure to certain carcinogenic heavy metals is the sum of the individual cancer risks.

Results and discussion
Operator exposure to heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) from ash, as well as the potential impact on their health was estimated using calculations based on the concentrations of heavy metals in wood pellet ash.
The concentrations of heavy metals in these samples are shown in Fig. 1. 4 As may be expected, there are significant differences in the concentrations of heavy metals depending on the type of wood pellet the ash of which was analysed. The content of heavy metals in wood pellets depends on the type of wood biomass from which the wood pellet is made, but also on the pollution caused by the production process and the use of chemically treated wood biomass. 15 Heavy metals are present in very small concentrations in woody biomass, although most of them are increased due to pollution. Such pollution could be due to the growth of wood in the urban, industrial zone of the city. Therefore, some European countries (Sweden, Denmark, Germany) have defined the permissible concentrations of heavy metals in the ash by legislation. 16 The exposure to heavy metals from ash is evaluated for non-carcinogenic (Hazard Index) and carcinogenic effects (Carcinogenic Risk Index).
Because different metals are commonly found in wood ash, humans, especially operators, are exposed to heavy metals via different pathways. If operators or owners of wood pellet stoves or fireplaces are not adequately protected, they are frequently exposed to the possible impacts of heavy metals from ashes during maintenance and cleaning.
Therefore, the calculation was realised for non-carcinogenic and carcinogenic health risks for three different routes: ingestion, inhalation, and dermal.
The obtained results for non-carcinogenic risk (HQ) for three different routes of exposure to metals are presented in Fig. 2. The calculated HQ values are less than 1. Thus, heavy metals from analysed ash posed no direct risk for operators.  Fig. 1 -Concentrations of heavy metals in ash of wood pellets. 4 Abbreviation d.w. refers to dry weight Slika 1 -Koncentracija teških metala u pepelu drvenih peleta. 4 Kratica d.w. označava suhu masu The highest HQ values were observed for ingestion (HQ ing = 3.62 • 10 −6 ); they were lower for dermal contact (HQ der = 7.18 • 10 −8 ), and lowest for inhalation (HQ inh = 1.77 • 10 −9 ). Our results for HQ values are consistent with the results of Kicinska 6 and Bourliva 17 . Therefore, the most relevant exposure pathway in terms of health concerns appears to be ingestion. This can be related to the fact that the actual daily intake was determined by the value of the hazard quotient in each pathway, and parameters used to calculate the exposure dose for each exposure pathway were different. 18 The total hazard index for non-carcinogenic substances was 3.70 • 10 -6 ( Table 3). HI < 1 suggests a very low risk for adverse health impact on operators exposed to wood pellet ash during cleaning of pellet stoves in confined environments. The fact that the greatest values for HQ (ingestion, inhalation, and dermal) are calculated for Pb (1.73 • 10 −6 ) and Cd (1.60 • 10 −6 ) can be concerning, even while the result for HI was within the acceptable levels. Therefore, operators may be exposed to the harmful effects of Pb and Cd from ash when maintaining and cleaning wood pellet stoves if they do not wear appropriate protective equipment. Pb is one of the most common heavy metals found in different types of ashes. 19 It is moderately toxic, it affects the intestine and central nervous system when swallowed, and causes anemia. 20 Cd is an element that has no function in the body. It is a cumulative poison that enters the body, is deposited in the kidneys, and remains almost a lifetime. Lung cancer, prostatic proliferative lesions, bone fractures, kidney dysfunction, and hypertension are all possible side effects of chronic Cd exposure. 21 Therefore, the permissible concentration of Cd in ash is defined in some countries, and is 30 and 20 µg g −1 for Sweden and Denmark, respectively. 22 Considering the cancer risk, the lowest CR value was calculated for Pb and inhalation (1.22 • 10 −13 ), and the high-est for Ni and ingestion (6.67 • 10 −10 ). Chronic exposure to small doses of Ni, especially through inhalation or ingestion of its dust, can cause cancer.

Conclusion
Heavy metals in ashes, like other hazardous substances, cause serious health consequences. Operators who clean and maintain stoves on a daily basis may be exposed to heavy metals from the ash produced by burning wood pellets. Therefore, a health risk assessment was performed for the three routes of exposure: ingestion, inhalation, and dermal exposure. In general, HQ and HI were lower than the permitted level of 1. The highest values for HQ were obtained for Pb and Cd, which are toxic elements, so further monitoring is required. In addition, operators and owners should use adequate protective equipment. The obtained result shows that the cancer risk levels of Ni, Cd, Pb, and Cr are lower than the acceptable range, indicating that the carcinogenic risks of mentioned metals for indoor wood pellet ash are negligible. Consequently, based on the results of the non-carcinogenic and carcinogenic risks for the metals analysed in the current study, the health risks from wood pellet ash for operators in indoor environments are not significant.