Diesel vs Clean Diesel: A Visual Comparison of Cityscapes

Diesel vs. Clean Diesel: How Much Healthier Are Today's Engines?

Samir D’Costa in Climate & Environment December 2025 • 3 min read.

"A deep dive into how modern diesel engines stack up against their older counterparts, exploring the science and regulations that drive cleaner air."

For over a century, diesel engines have been the workhorses of industry and transportation. But these engines have also been a source of public concern due to the harmful health effects associated with their emissions. These concerns have spurred significant advancements in emission control technologies and stricter regulations.

The introduction of new standards by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) in 2001 marked a turning point. These regulations pushed manufacturers to innovate, leading to the development of new technology diesel engines (NTDEs), often referred to as 2007-compliant diesel engines.

So, how do these modern engines compare to their older, pre-2007 counterparts? This article explores the findings of the Advanced Collaborative Emissions Study (ACES) and other research to provide a clearer picture of the impact of these technological advancements on our health and environment.

Pre-2007 Diesel Engines: A Cloud of Health Concerns

Diesel vs Clean Diesel: A Visual Comparison of Cityscapes

Before the 2007 regulations, diesel engines emitted significantly higher levels of particulate matter (PM) and nitrogen oxides (NOx), both of which pose serious health risks. Studies on pre-2007 diesel exhaust (DE) revealed a range of concerning effects.

Early research focused on diesel exhaust particulate (DEP) extracts. Kotin et al.'s studies in the 1950s demonstrated that DEP extracts were both toxic and carcinogenic in mice, with different strains showing varying susceptibility. The studies showed:

DEP extracts could induce tumors when applied to the skin of mice.
The toxicity and carcinogenicity of DEP varied depending on the animal strain and sex.
Even low levels of DEP could cause harm over time.

Further studies, such as those by Huisingh et al., identified that the mutagenic components of DEP were primarily found in the neutral fraction, containing polynuclear aromatics, phenols, ethers, and ketones. These findings highlighted the potential of DEP to cause DNA mutations.

The Road Ahead: Cleaner Air for Future Generations

The transition to 2007-compliant diesel engines represents a significant step forward in reducing the harmful effects of diesel emissions. The ACES findings, combined with other research, suggest that these new technologies have effectively reduced measurable tissue damage associated with reactive oxygen species.

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Everything You Need To Know

What prompted the development of new technology diesel engines (NTDEs), and what are they often called?

The introduction of new standards by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) in 2001 prompted the development of new technology diesel engines (NTDEs). These engines are often referred to as 2007-compliant diesel engines. These regulations pushed manufacturers to innovate to meet stricter emissions standards, particularly concerning particulate matter (PM) and nitrogen oxides (NOx).

What were the primary health concerns associated with pre-2007 diesel engines, and what specific pollutants were the main culprits?

The primary health concerns associated with pre-2007 diesel engines revolved around their high emissions of particulate matter (PM) and nitrogen oxides (NOx). Particulate matter, especially diesel exhaust particulate (DEP), was shown to be toxic and carcinogenic in animal studies. Nitrogen oxides contribute to respiratory problems and smog formation. Addressing these pollutants was central to the regulatory changes and technological advancements that led to cleaner diesel engines.

Can you elaborate on the Advanced Collaborative Emissions Study (ACES) and its significance in understanding the impact of 2007-compliant diesel engines?

The Advanced Collaborative Emissions Study (ACES) is a research initiative designed to assess the impact of new technology diesel engines (NTDEs) on health and the environment. ACES provides data on the differences between older and newer diesel engines, particularly regarding the reduction of harmful emissions and their associated health effects. It highlights the effectiveness of 2007-compliant diesel engines in reducing measurable tissue damage associated with reactive oxygen species, indicating a step forward in mitigating the adverse health impacts of diesel exhaust.

What specific findings from early research on diesel exhaust particulate (DEP) extracts raised concerns about the health impacts of pre-2007 diesel engines?

Early research on diesel exhaust particulate (DEP) extracts, such as the studies by Kotin et al. in the 1950s, demonstrated that DEP extracts were both toxic and carcinogenic in mice. These studies showed that DEP extracts could induce tumors when applied to the skin of mice, and the toxicity and carcinogenicity of DEP varied depending on the animal strain and sex. Even low levels of DEP could cause harm over time. Further studies, such as those by Huisingh et al., identified that the mutagenic components of DEP were primarily found in the neutral fraction, containing polynuclear aromatics, phenols, ethers, and ketones, highlighting the potential of DEP to cause DNA mutations.

In what ways did the 2007-compliant diesel engines improve upon pre-2007 models, and how did this impact air quality and health?

2007-compliant diesel engines improved upon pre-2007 models by significantly reducing emissions of particulate matter (PM) and nitrogen oxides (NOx). These improvements were driven by regulations from the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), which pushed manufacturers to innovate. The reduction in PM and NOx emissions leads to cleaner air, decreasing the risk of respiratory problems, cardiovascular issues, and other health concerns associated with diesel exhaust. The Advanced Collaborative Emissions Study (ACES) suggests that these new technologies have effectively reduced measurable tissue damage associated with reactive oxygen species. While the text does not cover the specific technologies used to achieve this, generally, diesel particulate filters (DPFs) and selective catalytic reduction (SCR) systems are employed in these engines.