This study combines an analysis of air pollutant trends in Iran (2006-2023) with a comparative analysis of CO₂ emissions across Iran, the United States, Canada, and the United Kingdom (2000-2022). Air pollution poses a significant global challenge, with transportation being a major contributor to deteriorating air quality and climate change. Globally, the transportation sector accounts for approximately 32% of CO2 emissions and 27% of total greenhouse gases (GHGs) [1] . In regions like the European Union, road transport consumes 81% of the sector’s energy and generates over 72% of its GHG emissions, far outpacing contributions from air (13.3%), marine (12.8%), and rail (0.5%) transport [1] . Similarly, transportation consumes nearly half of all fossil fuels worldwide, contributing about one-quarter of CO₂ emissions from fossil fuel combustion [2] . Notably, while Canadians constitute only 0.5% of the global population, Canada’s share of global GHG emissions is approximately 2% [3] . However, between 1990 and 2010, while Canada’s per capita GHG emissions decreased by nearly 5%, its total GHG emissions grew by 17% [3] . In the United States, transportation has surpassed the power sector since 2016 to become the leading GHG emitter [4] , with passenger light-duty vehicles alone responsible for nearly 60% of the sector’s emissions [4] . These statistics underscore transportation’s critical role in global emissions, a focus central to this study’s examination of air pollutants in Iran and CO₂ trends across Iran, the United States, Canada, and the United Kingdom.

Transportation’s environmental footprint extends beyond CO₂, encompassing a suite of air pollutants—CH₄, N₂O, HC, SOx, NOx, CO, and particulate matter (SPM)—that threaten both climate stability and public health. Road transport, in particular, dominates emissions profiles [5] , contributing roughly 20% of global CO₂ emissions [5] . In India, for example, motorized traffic is projected to increase energy demand and carbon emissions fivefold by 2020 from 2000 levels [2] . Exacerbated by traffic congestion that amplifies emissions through increased fuel use during idling and stop-and-go conditions [2] . Unlike the power sector, which has curbed emissions via renewable energy shifts [4] , transportation faces persistent barriers, including regulatory inconsistencies [4] and the lack of widespread GHG-free fuel alternatives [4] . Understanding these patterns and variations over time is essential for crafting effective mitigation strategies. Such long-term analyses are crucial for understanding the evolving environmental landscape in rapidly industrializing nations, particularly where the interplay of economic growth and environmental policy can lead to complex emission dynamics.

This study’s primary objective is to assess changes in air pollutant concentrations and identify significant trends across these regions and periods. For Iran, we analyzed greenhouse gases (CO₂, CH₄, N₂O) alongside SPM, HC, SOx, NOx, and CO from 2006 to 2023, revealing notable increases, particularly in GHGs. For the broader comparison, we reviewed CO₂ emissions from 2000 to 2022 across four nations, highlighting transportation’s role in shaping these trends. To ensure robust analysis, we employed a rigorous statistical methodology. Data normality was confirmed using the Kolmogorov-Smirnov test, followed by Levene’s test to assess variance equality. Where variances were unequal (sig < 0.05), the Brown-Forsythe test was applied, and Dunnett’s T3 test facilitated mean comparisons, providing a solid foundation for evaluating pollutant variations across timeframes and regions. This dual focus on Iran’s diverse pollutants and multi-national CO₂ trends addresses the urgent need to mitigate transportation-related emissions, offering insights into both local and global environmental challenges.

This study’s methodology was designed to rigorously analyze air pollutant gases in Iran from 2006 to 2023 and CO₂ emissions across Iran, the United States, Canada, and the United Kingdom from 2000 to 2022, ensuring a comprehensive and statistically robust evaluation. The approach encompasses data collection from diverse, credible sources and employs a combination of statistical tests and comparative emission estimation techniques to validate findings and contextualize transportation’s role in emission trends.

To conduct a comprehensive analysis of air pollutant trends, data were meticulously collected from reputable platforms and official national and international records. The datasets encompassed a broad spectrum of air pollutants, including greenhouse gases (GHGs: CO₂, CH₄, N₂O), sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO), and suspended particulate matter (SPM), as well as general CO₂ emissions.

For the analysis of air pollutant concentrations in Iran, annual data spanning from 2006 to 2023 were acquired. This dataset comprised yearly concentrations of GHGs (CO₂, CH₄, N₂O), SOx, NOx, CO, and SPM. For the broader comparative analysis of CO₂ emissions, yearly total data for Iran, the United States, Canada, and the United Kingdom were collected covering the period from 2000 to 2022.

The data collection relied on the following key sources:

Collectively, these diverse and credible sources enabled a robust multi-national comparison, leveraging both open-access platforms and authoritative governmental data to effectively reflect the influences of transportation, industrial activities, and other sectors on emission trends over the specified periods.

This section systematically outlines the statistical methodologies applied and presents the key findings derived from the analysis of air pollutant trends in Iran and comparative CO₂ emissions across multiple nations.

This study provides an updated analysis of air pollutant trends in Iran from 2006 to 2023 and a comparative examination of CO₂ emissions across Iran, the USA, Canada, and the UK from 2000 to 2022. The findings reveal critical shifts in pollutant concentrations and offer insights into both local and global environmental challenges.

Our updated analysis for Iran (2006-2023) indicates a significant increase in the average production of all measured pollutants when comparing the periods of 2006-2014 and 2015-2023, apart from NO_x. Specifically, greenhouse gases (GHGs), CH₄, N₂O, HC, CO, and CO₂ all showed a statistically significant rise in the latter period (2015-2023) compared to 2006-2014, with higher average concentrations. Notably, SPM and SOx also showed a statistically significant difference between the two periods; SPM production was significantly higher in the earlier period (2006-2014), while SOx levels also differed significantly. In contrast, NOx production showed no statistically significant difference between 2006-2014 and 2015-2023, suggesting levels remained relatively consistent across the timeframes, with insufficient evidence to conclude a statistically significant change.

The observed sharp rise in CO₂ in Iran, particularly from 2015 onwards, mirrors global industrialization and urban growth [1] . This surge is strongly driven by the transportation sector, including often overlooked factors such as Tehran’s waste fleet, which contributes significantly to national emissions [6] . While Iran’s road transport CO₂ emissions are lower than those of larger economies like the USA or China, the trajectory suggests a pattern akin to emerging economies [5] . The significant increases in CH₄ and N₂O highlight a growing concern in waste and agricultural management practices in Iran, pointing to areas requiring urgent attention and revised strategies [1] . Similarly, the rising trends in HC and CO may indicate an increase in vehicle activity, a greater reliance on older vehicle technologies, or inadequate fuel quality and emission control standards within the transportation sector. These findings underscore the need for targeted interventions beyond traditional CO₂ reduction efforts to manage a broader spectrum of air pollutants.

The contrasting behavior of SPM and SOx, which exhibited significant differences between the two periods, might suggest the influence of specific local controls, industrial shifts, or variations in fuel composition. Conversely, the stability in NOx levels could imply that existing emission control measures for this pollutant have been relatively consistent or that its primary sources have not seen substantial changes over the analyzed periods.

Globally, the United States continues to show the highest CO₂ emissions among the comparative countries, with transportation being a dominant contributor [4] . Canada’s lower GHG share, despite its large landmass and reliance on vehicle transport, reflects different factors, including policies designed to spur innovation in GHG mitigation and green technologies [3] . The European Union’s pronounced road transport GHG dominance (72% of its sector’s emissions) highlights challenges that resonate with Iran’s situation [1] . The comparison of CO₂ emissions across these nations between 2006-2014 and 2015-2023 confirms a statistically significant difference in CO₂ production among countries and across these time periods. This multi-national perspective contextualizes Iran’s emission trends within broader global patterns, demonstrating that while local factors are paramount, global trends and varied national responses provide crucial lessons.

Mitigation strategies are urgently needed to address these trends. Successful approaches, such as the impact of U.S. fuel standards on CO₂ reductions (though susceptible to policy rollbacks), offer valuable lessons [4] . For Iran, promising avenues include biogas swap initiatives, which could significantly reduce waste-related CO₂ emissions [6] . Furthermore, transit solutions that have led to projected GHG reductions, like Russia’s 12.7% cut by 2030 through transport optimization, provide robust statistical affirmation for these paths [7] . Power sector upgrades, such as natural gas combined cycles cutting Canada’s emissions by 59% and Iran’s by 32%, or CO₂ capture achieving 94% and 90% reductions, provide proven paths forward for reducing greenhouse gas emissions [8] . Policy recommendations must extend beyond the transportation sector to include comprehensive strategies for industrial emissions, waste management, and agricultural practices, particularly to curb the rising CH₄ and N₂O levels. Implementing stricter vehicle emission standards, promoting public transportation, encouraging electric vehicle adoption, and investing in renewable energy sources for both transportation and the broader energy sector are crucial. These targeted interventions, informed by the observed trends and comparative insights, are essential for balancing rising GHG emissions with effective pollutant controls, paving the way for a cleaner future. Future research should focus on probing the specific drivers behind the observed changes in pollutant levels and assessing the health and environmental impacts of these trends. Exploring sustainable alternatives, such as rail and marine transport, and refining emission estimation tools are also crucial next steps.

This study provided an updated analysis of air pollutant trends in Iran from 2006 to 2023 and a comparative examination of CO₂ emissions across Iran, the USA, Canada, and the UK from 2000 to 2022. Our findings reveal significant increases in most Iranian air pollutants, including GHGs, CH₄, N₂O, HC, CO, CO₂, SPM, and SOx, between the periods of 2006-2014 and 2015-2023, with only NOx showing no statistically significant change. These trends underscore the complex impacts of industrialization, urbanization, and specific sectoral activities, particularly transportation and waste management, on Iran’s air quality [1] .

The global CO₂ comparison highlighted the USA as the highest emitter, contextualizing Iran’s emission trajectory within broader international patterns and emphasizing the universal need for proactive strategies [4] [6] .

The robustness of these findings is supported by a rigorous statistical methodology. This research establishes a critical foundation for developing and implementing targeted interventions, such as enhancing vehicle emission standards, promoting public transport, optimizing waste management, and fostering sustainable energy transitions across all sectors, to achieve a cleaner and more sustainable future. Future research should delve deeper into the specific drivers of these changes and their precise health and environmental impacts, while also exploring sustainable transport alternatives.