A Systematic Review on Concentration of Heavy Metal in the Ambient Air of Different Industries and the Health Risk Assessment

Introduction

Human uses the air every moment, it has a vital function for living beings [1]. A person breathes about 22,000 times a day and about 25 kg of air enters the lungs [1]. Generally, the effects of air pollution on humans are considered, however, air pollution is not ending of these effects [2]. The pollutants can affect humans, animals, plants and materials or the global environment and leave adverse effects [2]. Many of the pollutants enter the atmosphere via sources that are currently not controlled enough [3]. One of the pollutants is heavy metals [3]. Heavy metals are metals including cobalt, chromium, copper, gold, iron, lead, zinc, cadmium, manganese, nickel, mercury, and a limited number of other elements which have a specific gravity greater than 6 g/cm3 or an atomic mass greater than 50 [4]. From an economic point of view, they have an outstanding strategic role [4]. Although the presence of some of these elements in the body is very important in terms of nutrition for the metabolism, facing their excessive concentrations through different sources (air, water, and soil), can have adverse effects [5]. From the point of view of occupational health, exposure to heavy metals is also of particular importance. Occupational exposure to heavy metals mainly occurs through inhalation and during various industrial processes [6]. It can be mentioned that the effects on the central and peripheral nervous system, digestive system, kidneys, hematopoietic system, cardiovascular system, enzyme disorders, and finally cancer are the major effects of exposure to heavy metals [7]. Today, due to the industrialization of current societies, many cities or megacities have many industrial factories. As mentioned, one of the pollutants emitted from industries is heavy metals [8]. People in the cities, the workers of the industries, and their families are exposed to the metals in various ways. These metals enter workers’ houses through the contaminated clothes, hair and shoes, and expose their wives and children [9]. In this article, a systematic review of the concentration of heavy metals in the indoor air of the industries was investigated and the related health risks were calculated for the exposed workers of industries.

**Study Method**

In this systematic review, Databases of Science Direct, Google Scholar, SID, and MagIran were searched by applying the keywords “Air,” “Heavy metals”, and “Industry” in the period time from October 14, 2022, to October 23, 2022. In the next step, from the resulting articles, the cases including “Air,” Heavy metals”, and “Industry” in their title or summary keywords were selected. In this regard, 193 articles were related to the topic and 20 of them were selected after study of the abstract. Finally, 9 articles that reported the concentration of heavy metals in industrial air were included in the study [1, 2, 3, 4, 5, 6, 7, 8, 9].

To obtain the carcinogenic and non-carcinogenic risk, we use the method reported by Rostami, et al. [10]. Calculations, while it was 30 years for carcinogenic risk assessment in this line of research, equations (1) and (2) were used for calculation of the exposure concentration (EC) and estimated daily intake (EDI) then they were used for estimation of carcinogenic and non-carcinogenic risk, respectively.

Where, c: The concentration of employees exposed to heavy metal, ET (exposure time): In various industries, workers shifts are set every 8 hours., ED (exposure duration): The retirement age of workers is 30 years, EF (exposure frequency): The number of days that workers work in a year, which actually has 52 weeks in a year, each week including 5 working days, so we put 260, AT (averaging time): ED × 365 days, BW (body weight): The usual weight for women workers is 70 kg and for men 80 kg, so we use the average of these two, which is 75, and IR (inhalation rate): A normal person breathes 16 m³/day of air for 8 hours a day, but a worker uses 20 m³/day due to physical activities during our work. To assess the carcinogenic risks of heavy metals, we followed the equation provided by The United States Environmental Protection Agency (USEPA).

$$EC \left( \frac{\mu g}{m^3} \right) = \frac{c \left( \frac{\mu g}{m^3} \right) \times ET \left( \frac{h}{day} \right) \times ED(year) \times EF \left( \frac{day}{year} \right)}{AT(year) \times 365 \left( \frac{day}{year} \right) \times 24 \left( \frac{h}{day} \right)}$$

$$EDI \left( \frac{mg}{kg.day} \right) = \frac{C \left( \frac{\mu g}{m^3} \right) \times IR \left( \frac{m^3}{day} \right) \times ET \left( \frac{h}{day} \right) \times ED(year) \times EF \left( \frac{day}{year} \right)}{AT(year) \times 365 \left( \frac{day}{year} \right) \times 24 \left( \frac{h}{day} \right) \times BW(kg)}$$

Where, the Lifetime Cancer Risk (LTCR) denotes inhalation lifetime cancer risk and the Unit Risk (UR) denotes cancer unit risk. The UR values of As, Cd, Cr, and Ni are 4.3 × 10⁻³, 1.8 × 10⁻³, 1.2 × 10⁻², and 4.8 × 10⁻⁴ (µg/m³)⁻¹, respectively. Also, to assess the non-carcinogenic risk of Cr, Hg and Mn, the hazard quotient (HQ) value was employed as shown in the following equation (4).

To calculate the reference dose (RfD) for the heavy metals, we employed the reference concentration (RfC) approach. The values of RfC for Cr, Hg, and Mn were 8.86 × 10⁻⁴, 3.0 × 10⁻⁴, and 5.0 × 10⁻⁵, respectively.

While HQ values of more than 1.0 indicate a potential risk for exposed people a value of less than 1.0 is an indicator of an unacceptable level of risk.

$$LTCR = EC \left( \frac{\mu g}{m^3} \right) \times UR \left( \frac{\mu g}{m^3} \right)^{-1}$$

$$HQ = \frac{EDI \left( mg/kg.day \right)}{RfD \left( m/kg.day \right)}$$

Conclusion

According to the obtained results, attention should be paid to the cement industries and the zinc smelting industry because the observed concentrations of metals in the air of these industries are much higher than other industries. In accord with the results obtained from the calculation of LTCR and HQ, as well as according to the stated standard, the LTCR of chromium metal exceeds the permissible limit and should be taken into account. We should also pay more attention to chromium metal and arsenic in the cement industry and to cadmium metal in the industry. It should be noted that the Atomic Energy Organization has also allocated a significant number in the LTCR. Regarding the results, more rigorous monitoring and health protection instructions are required to control of the adverse effects of the heavy metals and the workplaces. In this regard, emphasis on application of appropriate personal protection equipment and decrease of the contact time with the considerate shift planning by the managements system can be helpful.

Acknowledgments

The authors are grateful to thank you Semnan University of medical sciences for supporting this research and thank you to Mr. Mohamad Javad Akbari for typing quotes from this article.