Estimation of Carbon Monoxide, Sulfur Oxides, Nitrogen Oxides, Volatile Organic Compounds, and Particulate Matters Emission Due to Cryptocurrency Miners’ Activity in Iran

: Nowadays, electricity consumption has increased worldwide due to the activity of cryptocurrency miners. Much of Iran’s electricity is generated by fossil fuel power plants. So, generating more electricity means producing more air pollutants in Iran. There is not sufﬁcient information about the effects of cryptocurrency mining on Iran’s air pollution. This study aims to estimate the amount of carbon monoxide (CO), sulfur oxides (SO x ), nitrogen oxides (NO x ), volatile organic compounds (VOCs), and particulate matter (PM) emitted by Iran’s power plants when they generate extra electricity for cryptocurrency miners. In this study, we ﬁrstly estimated the amount of fuel used for the electricity needed for cryptocurrency miners. Then, the amounts of emitted NO x , CO, VOCs, SO x , and total PM for generation of such electricity were estimated via the guidelines of the European Environment Agency for emission inventory estimation. The results showed that an on average of 3530, 1547, 103, 11, and 35 tons of NO x , CO, VOCs, SO x , and total PM, respectively, have been emitted into the atmosphere in Iran annually.


Introduction
As a result of fast industrialization and economic expansion, air pollution is growing. The rising use of fossil fuels, the proliferation of motor vehicles, the growth of global aviation operations, shipping, and building are all contributing to the rise in air pollution concentrations. Cryptocurrency mining has recently been chastised for its carbon footprint [1]. Higher levels of air pollution hurt human and animal health, increasing the risk of skin, heart, brain, and reproductive system disorders [2]. Pollutants in the air also wreak havoc on regulatory immune responses [3]. Therefore, it is necessary to find a rapid solution to reduce the air pollution problem.
Cryptocurrencies are a type of digital asset and are usually managed in a decentralized manner [4]. One of the first famous cryptocurrencies was Bitcoin, which was created and released in 2009 by a person using the pseudonym Satoshi Nakamoto [5]. After Bitcoin, many more cryptocurrencies, such as Ethereum and Tether, were established. The current statistics say that between 4.5 and 10% of the world's mining for cryptocurrencies is performed in Iran [6,7]. A large amount of electricity is required to mine cryptocurrencies [8].
It is estimated that about 15,000 MW of electricity is used worldwide for the mining of cryptocurrencies [9]. To legally extract cryptocurrencies in Iran, a license must be obtained from the Ministry of Energy of Iran. The electricity used to mine cryptocurrencies in Iran is divided into two parts: legal and illegal cryptocurrency mining.
There are different estimations for the total electricity used for the extraction of cryptocurrencies in Iran. Up to May 2021, the Ministry of Energy of Iran has already issued Earth 2021, 2 licenses to supply 320 MW of cryptocurrency mining units [9]. The reports show that only some of licensed units for cryptocurrency mining have currently been activated, and between them they consume around 200 MW of electricity [9]. The amount of electricity consumed for legal cryptocurrency mining in Iran is estimated to be between 200 and 300 MW (on average 250 MW) [7,9]. Additionally, the consumption of electricity for illegal cryptocurrency mining in this country is between 400 and 1200 MW (on average 800 MW) [7,9]. It seems that the total electricity used for both the legal and illegal mining of cryptocurrencies in Iran is from 600 to 1500 MW (on average 1050 MW). The average world cost of electricity is about 13.31 cents per kWh and it means that nearly USD 80,000,000 is used annually to mine various cryptocurrencies in Iran. Although many different means are applied to generate electricity, the most important method is the use of power plants that consume fossil fuel energies. Many and varied air pollutants, such as carbon dioxide CO 2 , CO, SO x , NO x , VOCs, and PM, are generally emitted into the atmosphere during the burning of fossil fuels in power plants. Consequently, there is a direct relationship between electricity usage and the emission of air pollutants. Nowadays, the amount of electricity usage is increased due to cryptocurrency miners' activities. Power plants should increase the amount of electricity generation to provide the extra electricity needed for cryptocurrency miners. More electricity generation means more emission of air pollutants. It was estimated that the indirect CO 2 emission of cryptocurrency miners is around 80.43 million tons of CO 2 , which is 0.24% of global CO 2 emissions [10]. Although many studies can be found on the air pollution status in Iran, the relationship between the mining of cryptocurrencies in Iran and air pollution has not been investigated yet. Additionally, there is no scientific estimation of the total amount of air pollutants that are emitted into the atmosphere due to the activities of cryptocurrency miners in Iran. In this study, the amounts of CO, SO x , NO x , VOCs, and PM emitted from total Iranian fossil fuel power plants to generate the electricity required for the mining of cryptocurrencies has been estimated. The results of this study can help the Iranian government to decide whether the limitation or expansion of cryptocurrency mining in Iran is needed.

Methodology
The reports showed that 25.4% of electricity in Iran is generated by renewable energies or without fossil fuels usage in combined cycle power plants, which means that they do not emit any air pollutants [11]. On the other hand, about 74.6% of electricity is directly generated using fossil fuel power plants [11]. The majority of Iran's fossil fuel power plants use natural gas as their energy source. However, in some cases, within short periods other fuels, such as diesel fuel and mazut, are replaced with natural gas in some power plants, thereby increasing the amount of air pollutants emitted. It was reported that only around 6 power plants out of 123 sometimes use mazut as their energy source [12]. Another report shows that nearly 6% of electricity in Iran is generated by mazut [13]. Additionally, only 0.5% of the total amount of electricity generated in Iran is produced by diesel power plants [14]. This information shows that the most significant fuel used in Iran's power plants is natural gas. The Ministry of Energy of the Islamic Republic of Iran is currently trying to reduce the amount of time it is taking to of replace mazut with natural gas in power plants to decrease the quantity of air pollutants emitted. Between 21 March 2019 and 21 March 2020, 6 billion liters of diesel fuel, 3.5 billion liters of mazut, and 67 billion m 3 of natural gas were consumed in Iran's thermal power plants [13,15]. In this study, it is assumed the periods in which that power plants use diesel fuel and mazut as energy sources are too short to have a significant effect on the quantity of air pollutants emitted at the national level. Approximately 0.37% to 0.57% of total natural gas has been used for the generation of electricity for the activities of legal cryptocurrency miners. Additionally, 0.74% to 2.22% of total natural gas has been used for the activities of illegal cryptocurrency miners. It can be concluded that the total natural gas used to mine cryptocurrencies in Iran should be between 1.11% and 2.78%. Table 1 shows the amount of natural gas that is used for both illegal and legal cryptocurrency miners. The total quantity of natural gas used to generate electricity for cryptocurrency miners in Iran was estimated to be between 844 and 2111 million m 3 . Nearly 563 to 1689 million m 3 and 281 to 422 million m 3 of natural gas were used to generate electricity for illegal and legal cryptocurrency miners, respectively. where C Air pollutant is the emission rate of a certain air pollutant in g, F is the amount of fuel energy used for the generation of electricity needed by cryptocurrency miners in GJ, and EF Pollutant is the emission factor of a particular air pollutant in g/GJ, and 0.746 is the share of fossil fuel electricity energy in Iran. Based on the statistics employed by the European Environment Agency, if a power plant operates with natural gas as its fuel, the emission factors are as follows: for NO x , 89 g/GJ of energy produced; CO is 39 g/GJ of energy produced; VOCs are 2.6 g/GJ of energy produced; SO x constitutes 0.281 g/GJ of energy production; PM is 0.89 g/GJ of energy produced; PM 10 0.89 g/GJ of energy produced and PM 2.5 0.89 g/GJ of energy produced [16]. The above emission factors are for situations where the power plant is not equipped with air pollution control systems. In this study, it is assumed that none of Iran's power plants are equipped with air pollution control systems. Each m 3 of natural gas has 8600 Kcalories energy [17]. In total, Iran's power plants used 67 billion m 3 of natural gas, which means 57.62×10 13 Kcalories energy. However, only 1.11% to 2.78% of natural gas is used to mine cryptocurrencies in Iran; therefore, the total electrical energy used by cryptocurrency miners in Iran can be estimated to be from 7.26222 × 10 12 to 1.81556 × 10 13 Kcalories, which is equivalent to 30 to 76 gigajoule (GJ) (it should be noted that each kilocalorie is equal to 4184 Joule). Then, the estimated electrical energy was replaced instead of F in Equation (1). Additionally, the emission factor for each air pollutant was replaced instead with EF Pollutant . Then, the amount of C Air pollutant was calculated for each air pollutant.

Results and Discussion
The results show that 2020 to 5040 tons (on average 3530 tons) of NO x are annually emitted into the atmosphere due to the legal and illegal activity of cryptocurrency miners in Iran, and that 672 to 1010 (on average 841 tons) and 1340 to 4030 tons (on average 2690 tons) of this emitted NO x are due to legal and illegal activities, respectively (see Figure 1). NO x are chemical molecules created when oxygen and nitrogen react with each other at high temperatures, primarily during the combustion of fuels such as oil, diesel, gas, and organic waste. Combustion processes in the electrical power plants are a suitable place to form NO x . NO x has both direct and indirect health impacts on humans. Breathing difficulties, headaches, persistently decreased lung function, eye discomfort, lack of appetite, and corroded teeth are all possible side effects. It can have an indirect impact on humans by destroying the ecosystems both in the seas and on land, killing animals and plants [18].  Figure 2 shows that 2.1 to 3.1 tons (on average 2.7 tons), and 4.2 to 12.7 tons (on average 8 tons) of leagal and illeagal SO x are annually emitted into the atmosphere, respectively, due to cryptocurrency miners' activity in Iran. High levels of SO x can irritate the eyes nose, throat, and lungs and inflame the respiratory system, especially during strenuous physical exercise [19].   1179 tons), respectively. CO poisoning may happen quickly and without warning, causing loss of consciousness and subsequent suffocation. CO poisoning can cause headaches, tiredness, dizziness, sleepiness, and nausea as well as constriction across the chest. Angina patients may experience sudden chest discomfort [20].
The results illustrate that 58 to 147 tons (on average 103 tons) of VOCs are also annually emitted into the atmosphere of Iran by generating electricity for use by cryptocurrency miners. The amounts of legal and illegal VOCs emission caused by the activities of cryptocurrency miners are 6 to 10 (on average 8 tons) and 13 to 40 (on average 27 tons), respectively (see Figure 4). Organic substances with a high vapor pressure at room temperature are known as VOCs. They have a low boiling point that permits them to evaporate quickly. Most of the VOCs are dangerous for human health [21].  The average total emissions of PM are estimated to be between 20 to 50 tons (on average 35 tons). The amounts of PM emitted due to electricity use via legal and illegal cryptocurrency miners in Iran are 6 to 10 tons (on average 8 tons) and 13 to 40 tons (on average 35 tons), respectively (see Figure 5). PM inhalation can cause early mortality in patients with heart or lung illness, nonfatal heart attacks, irregular heartbeat, worsened asthma, reduced lung function, and increased respiratory symptoms. It was reported that the yearly quantities of CO, VOCs, NO x , SO x , and PM 10 released by on-road mobile sources in Isfahan, one of the biggest cities in Iran, were 267,600, 12,600, 20,300, 300, and 2300 tons, respectively, in 2018 [22]. It seems that the amounts of NO x emissions due to cryptocurrency miners in Iran are nearly 9.9% to 24.8% of the total on-road mobile sources found in Isfahan. Moreover, the levels of CO, VOCs, SO x , and PM emitted due to cryptocurrency miners' activities in Iran are 0.82% to 3.3%, 0.45% to 1.16%, 0.7% to 1%, and 0.86% to 2.17%, respectively, of the quantities emitted in Isfahan from on-road mobile sources. The amounts of air pollutants emitted due to the activity of cryptocurrency miners in Iran are compared in Figure 6.

Conclusions
Cryptocurrency miners are distributed throughout Iran and utilize electricity across the country. Iranian power plants must generate extra electricity to support these cryptocurrency miners. The generation of this extra electricity causes the annual average emission of 3530, 1547, 103, 11, and 35 tons of NO x , CO, VOCs, SO x , and total PM, respectively, to be emitted into the atmosphere in Iran. It seems that the pollutants emitted due to both legal and illegal cryptocurrency miners' activities in Iran do not have a significant share in the air pollution observed in Iranian cities. The results of this study can help the Iranian government to decide whether the limitation or expansion of cryptocurrency mining in Iran is needed.  Data Availability Statement: All the data used in this study was collected from various Iran official news agencies. The data are accessible via their websites. The reference for each collected data including their URL has been provided in the reference section.

Acknowledgments:
The manuscript's authors thank the Jami Institute of Technology for supporting them to write this manuscript.

Conflicts of Interest:
The authors declare no conflict of interest. The authors are aware of and comply with best practices in publication ethics, specifically those about authorship (avoidance of guest authorship), dual submission, manipulation of figures, competing interests, and compliance with policies on research ethics. The authors adhere to the publication requirements that the submitted work is original and has not been published elsewhere in any language. Additionally, all procedures performed in studies involving human participants were by the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. All procedures performed in this study involving animals followed the ethical standards of the institution or practice at which the studies were conducted.