**2. Results**

In Table 1, the essential oil yields from three different frankincense gum cultivars, called Najdi, Sahli and Houjri, are shown. The samples were subjected to three successive hydrodistillations (2, 4, 6 h) to obtain three corresponding essential oils that were called Grade 1, Grade 2 and Grade 3. The essential oils from *B. sacra* oleogum resins were obtained in yields of 8.92–12% (*w*/*w*) in Grade 1. Regarding Grade 2 essential oils, Najdi recorded the highest percentage of 2.20% (*w*/*w*), followed by Sahli, of 2.08% (*w*/*w*). Grade 3 showed the lowest yield percentages of 0.40–0.72% (*w*/*w*) of essential oils (Table 1). However, Grade 1 essential oil samples recorded much higher yield values in comparison to values mentioned in the literature [11]. The essential yields of hydrodistilled *B. sacra* oleo gum resin are comparable to a previous study [10].

**Table 1.** Percentage yields of essential oils obtained from three different frankincense gum cultivars called Najdi, Sahli and Houjri, subjected to three successive hydrodistillations.

All the essential oil samples were investigated and compared for the presence of volatile components, see Table 2. The data obtained from gas chromatography–mass spectrometry (GC/MS) analysis of Grade 1 essential oil indicate the presence of more than 30 components. These chemical components were grouped into three categories, namely monoterpenes, diterpenes and sesquiterpenes.

**Table 2.** Chemical constituents identified in Grade 1, 2 and 3 (respectively 2, 4 and 6 h hydrodistillation time) oleogum resin essential oils. RI = retention index relative to C8–C36 *n*-alkanes on ZB-WAX column, MS = NIST and Wiley library and the literature, tr. = trace (<0.01%).

**Table 2.** *Cont*.

#### *2.1. Composition of Resin Essential Oils*

In the case of the cultivar Najdi, a total of 23 active components were identified in the Grade 1 essential oil by GC/MS analysis. The highest percentage was reported for α-pinene, followed by δ-3-carene, camphene and β-pinene (79.59%, 9.94%, 3.23%, and 2.39%, respectively). In the Grade 1 essential oil of Sahli, GC/MS data showed that 29 constituents were present in the essential oil. The major components were α-pinene (78.69%), sabinene (7.78%), camphene (2.66%), limonene (2.31%) and β-pinene (2.66%). In the case of Houjri, GC/MS reports 37 volatile components, which is the highest number compared to Najdi and Sahli with α-pinene, followed by sabinene, camphene, β-pinene, δ-3-carene and cis-verbenol (71.09%, 7.63%, 3.00%, 2.17%, 2.16% and 1.85%, respectively). Data are shown in Table 2.

In Grade 2 essential oil, a sensible reduction of monoterpenes was reported in comparison to Grade 1 essential oil. In all cultivars the major components α-pinene, camphene, β-pinene and δ-3-carene decrease, and the sesquiterpens β-elemene and β-eudesmene increase with a successive 4 h of hydrodistillation.

Grade 3 essential oils show a% reduction in monoterpens (α-pinene, camphene, β-pinene and δ-3-carene) in three cultivars, while at the same time the content in % of sesquiterpenes (β-elemene, β-eudesmene, γ-cadinene) is increased. The highest percentage was recorded for α-pinene. Table 2 shows the results.

#### *2.2. Antibacterial Activity of Essential Oils*

All the essential oils from the different frankincense cultivars were tested against two relevant pathogens, *S. aureus* and *P. aeruginosa*, included in the global priority list of antibiotic-resistant bacteria from WHO/OMS [12], and against some reference strains of dermatological interest: *Staphylococcus epidermidis* ATCC 12228, *Staphylococcus hominis* ATCC 27,844 and *Propionibacterium acnes* ATCC 11827. The results are expressed in terms of minimal inhibitory concentration (MIC) in percentage *v*/*v* or in mg/mL and are reported in Table 3. The results indicate that Grade 2 essential oil of Najdi showed an MIC of 52 mg/mL for *S. aureus* ATCC 6538, *P. aeruginosa* ATCC 9027 and *P. aeruginosa* ATCC15442. The essential oil of Grade 2 Sahli was active against all reference strains of *S.aureus* and *P. aeruginosa*, with MIC values ranging from 440 to 110 mg/mL. It showed the best MIC values, ranging from 6.16 to 0.264 mg/mL, against bacterial strains of dermatological interest *S. epidermidis*, *S. hominis* and *P. acnes*.

**Table 3.** Antibacterial activity of essential oils expressed in terms of minimum inhibitory concentration (MIC) in %*v*/*v* or mg/mL (between brackets).

#### *2.3. Antifungal Activity of Essential Oils*

The antifungal activity was determined against *C. albicans* ATCC 10,231 and *Malassezia furfur* ATCC 14521. All tested samples showed an interesting antifungal activity against *C. albicans* and *M. furfur*, with MIC values ranging from 54.56 to 0.240 mg/mL. In particular, Grade 2 essential oil from Najdi and Grade 1 essential oil from Sahli showed MIC values at the lowest tested concentration corresponding to a percentage *v*/*v* of 0.03 (≤0.252 mg/mL) against both fungal pathogens. The data were shown in the Table 4.

**Table 4.** Antifungal activity MIC values in %*v*/*v* and in mg/mL between brackets.


(3) The frontiers and trends of the Chinese and English literature are summarized through the analysis of burst keywords. The Chinese literature has obvious stage research characteristics, and this paper divides its evolution process into three stages: the early research frontier (1991–2005) stage has more burst keywords focusing on the choice of research methods, and other burst keywords show that the industries causing air pollution are also mainly focused on the petrochemical industry. The medium-term research frontier (2005–2013) focuses on the impact of indoor pollution on human health. The research themes of the latest research frontier (2013–2021) are mainly focused on air pollution caused by haze, the relationship between climate change and air pollution, and premature death caused by air pollution. The research frontiers in the English literature can be summarized into three main categories: types of atmospheric pollutants and pollution patterns, health economic effects of atmospheric pollution, and synergistic governance of atmospheric pollution and climate change. Here, each category has a significant stage evolution.

#### **6. Discussion**

This article provides a broad overview of existing research results studying the relationship between air pollution and health, which provide strong evidence that air pollution around the world has significant health effects, deepen our understanding of environmental issues, and serve as a cautionary tale for newly industrializing countries in the prevention of air pollution. Some of the findings have already begun to have policy implications, and we anticipate that research in this area is likely to make an increasing contribution to environmental policymaking in the relevant countries covered in the article.

Combining the above analysis in this paper and the related literature, we believe that several possible important directions for future research on the relationship between air pollution and health are still worth further deepening and expanding. First, studying the health consequences of sustained pollution exposure is one of the most important research topics for the future. How to collect credible evidence of the long-term effects of air pollution, how to break through the difficulty of identifying exogenous changes in long-term pollution exposure, and breakthroughs in research methods may have to be continued in depth in future studies.

Second, the negative impacts of air pollution are not only an issue in terms of direct physical health damage, but air pollution affects long-term human capital acquisition, human cognition, and productivity by affecting physical health in turn. Given this, the economic loss of health based on direct health consequence measures is greatly underestimated, and therefore the negative health impacts of air pollution should be further explored fully in future studies.

Third, through this paper, we can clearly find that PM2.5, PM10, and NO2 air pollutants still seriously affect public health, and we have to continue to make efforts for air pollution control. For example, we can continue to research the sources and control of air pollutants, air pollution policy formulation and policy evaluation, and air pollution synergistic governance. Among them, the synergistic management of air pollution and climate change is one of the latest research frontiers in this field. The common emission sources imply that there are many links between air pollution and climate change, and the implementation of climate policies can also reduce air pollution and thus benefit health, so the question concerning whether there is an optimal combination of air pollution and climate change policies is also worth further exploration by scientists.

**Author Contributions:** Y.G.: conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, writing—review and editing, visualization, supervision, project administration. S.Y.: conceptualization, methodology, software, validation, resources, writing—original draft, writing—review and editing, supervision, project administration, funding acquisition. Y.X.: writing—review and editing, supervision, project administration, software. N.W.: validation, supervision, writing—review and editing, methodology, software. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This paper takes the Chinese core journals in CNKI and the core collection of Science Citation retrieval Web of Science as the data source to make general measurement and statistics on the research on the relationship between air pollution and public health.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

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