## 4 Discussion  
This study was carried out in the Alborz province to evaluate the energy consumption and environmental pollution of lettuce production. The total energy consumption for lettuce production was 50,191.16 MJ/ha while the output energy of this system was 20,269.2 MJ/ha. The calculated energy indices indicated that lettuce production is not efficient. On the other hand, diesel fuel and chemical fertilizers had the highest energy consumption. Therefore, avoiding the use of old machines (to lower fuel consumption) and the use of the proper amount of chemical fertilizers can improve lettuce production. The share of nonrenewable energies in the production of lettuce was above 99%. The use of animal waste and green fertilizers can preserve soil fertility while decrementing environmental pollution. [PERSON], [PERSON] and [PERSON] [38] explored the energy consumption for lettuce production in the Mazandaran province of Iran and declared that the highest energy consumption was for diesel fuel (47%). In one recent study (2018), researchers stated that the main processes are responsible for 51% of GW potential and almost entirely responsible for abiotic discharge (99%); both are related to the use of fossil fuels during a jam-production process [41]. For the research that [PERSON] et al. (2020) performed in the grape industry, they concluded that, for freshwater ecotoxicity and terrestrial ecotoxicity, an important contribution to the effects was related to the release of pesticides into the environment, and that was one of the reasons it was considered as fuel consumption. Finally, due to fuel consumption, mechanical operations also had a very important contribution to several impact categories, including: ozone depletion, climate change, particulate matter formation, and photochemical oxidant formation [42]. Grapes are one of the most important agricultural products around the world, which have been widely researched [43; 44].  
The ER in the production of processed products, such as bread and sugar, have higher values (0.52) [45]. The EP was 17.28 kg/MJ, suggesting the production of 17.28 kg product when consuming one MJ of energy. The SE (0.06) showed an inverse relationship with the EP, in the research results of [PERSON] et al. [46], who reported energy efficiency in the production of coriander seeds as 0.06. Table 4 also indicates that about 56% of the consumed energy is direct, while 44% of that is indirect. The contribution of renewable energy is below 1%, so nonrenewable energy accounts for more than 99% of the consumed energy. [PERSON] and [PERSON] [47] reported the share of renewable energy in the  
Figure 4: Parts of the normalized effect in lettuce production.  
production of wheat flour as 99.19%, and so the amount of indirect and renewable energy was far more than nonrenewable and direct energy.  
It should be noted that in this study and other similar studies that are conducted with the approach of evaluating the life cycle of plant products, only the release of pollutants is examined, although plants can also absorb environmental pollutants. For example, plants absorb a great deal of carbon dioxide during their growth period, so the amount of carbon dioxide absorbed by the plant may be greater than its emission. The noteworthy point is that the agricultural sector is not the only emitter of this gas but it is the only absorber of this gas. In other words, plants have the ability to balance carbon in nature; therefore, due to the large volume of carbon dioxide emission from other sectors, the emission of pollutants in the agricultural sector should be minimized so that plants can establish a carbon balance.