Fossil-fuel use in transportation, industry, and power generation emits multiple harmful air pollutants beyond CO$_2$. Three major concerns are carbon monoxide (CO), hydrocarbons (especially VOCs), and particulate matter (PM), each with distinct sources, chemistry, and health effects.

Key pollutants from fossil-fuel combustion

Carbon monoxide (CO)

Major sources

• Motor vehicles (especially cold starts, idling, poor engine tuning)

• Faulty furnaces/boilers and other incomplete-burning equipment

• Biomass or fossil-fuel burning in enclosed or poorly ventilated spaces

Why it forms

• When there is insufficient oxygen or low combustion temperature, carbon is only partially oxidised.

• CO can persist long enough to be transported and inhaled, especially in high-traffic corridors and urban “street canyons.”

CO can later be oxidised in air to CO$_2$, reducing CO levels over time, but near sources it can reach harmful concentrations.

Health relevance

• CO is dangerous because it interferes with oxygen transport in blood, so exposure is most risky for people with cardiovascular disease, infants, and the elderly.

Oxyhemoglobin dissociation curves comparing normal blood to blood with substantial carboxyhemoglobin (CO-bound hemoglobin). The figure illustrates two key effects of CO exposure: fewer binding sites are available for $O_2$ (lower total $O_2$ content) and the remaining hemoglobin holds onto $O_2$ more tightly (left shift), reducing oxygen delivery to tissues. Source

• High exposure can cause headache, dizziness, and impaired judgement, increasing accident risk.

Hydrocarbons and VOCs

Major sources tied to fossil fuels

• Unburned fuel in vehicle exhaust (especially during inefficient combustion)

• Evaporation from gasoline during storage and fueling

• Industrial solvents and petrochemical processes

Why they matter for air quality

• Many hydrocarbons are toxic (e.g., benzene is a known carcinogen).

• VOCs contribute to the formation of secondary pollutants (pollutants formed in the atmosphere), which can worsen overall air quality even if VOCs themselves are emitted at moderate levels.

• Some VOCs create strong odors and reduce local quality of life near refineries or high-traffic areas.

Factors that increase hydrocarbon emissions

• Low combustion efficiency (poor mixing of fuel and oxygen)

• Cold engines and short trips

• Leaks during fuel handling and storage

Particulate matter (PM)

Major fossil-fuel-related sources

• Diesel and gasoline exhaust (soot/black carbon)

• Coal and oil combustion (fly ash, metal-containing particles)

• Secondary particle formation from gaseous emissions that transform into particles (for example, some organics can become particle-phase material)

Why particle size is critical

• PM$_{10}$ (≤10 μm) tends to deposit in the nose and upper airways.

• PM$_{2.5}$ (≤2.5 μm) can penetrate deep into the lungs and is more strongly linked to severe health outcomes.

Relative-size diagram for particulate matter, contrasting coarse particles (PM${10}$) with fine particles (PM${2.5}$). The visual helps connect particle diameter to deposition location in the respiratory system—smaller particles are more likely to reach deeper lung regions and contribute to more serious cardiopulmonary outcomes. Source

How these pollutants degrade air quality

Local to regional pollution patterns

• CO and many VOCs often peak near emission sources (traffic corridors, dense urban areas).

• PM can be both local (near roads/industrial stacks) and regional because fine particles remain airborne longer and can be transported.

EPA trend graphs summarizing measured ambient PM$_{2.5}$ concentrations over time from the national monitoring network. This provides a real-data example of how particulate pollution is quantified (as concentration) and how regional-scale transport and broad emission controls can influence long-term air-quality patterns. Source

Visibility and environmental effects

• Fine particles scatter and absorb light, producing haze and reducing visibility.

• Dark particles like black carbon can increase warming by absorbing sunlight and, when deposited on snow/ice, can reduce reflectivity and speed melting (a climate-relevant air-quality co-impact).

Human health impacts (core APES emphasis)

Carbon monoxide

• Reduces the body’s ability to deliver oxygen to tissues, stressing the heart and brain.

• Can be acutely dangerous in high concentrations, particularly indoors or in enclosed microenvironments (e.g., garages).

Hydrocarbons/VOCs

• Can irritate eyes and respiratory pathways.

• Some compounds are carcinogenic or otherwise toxic, depending on chemical structure and exposure duration.

Particulate matter

• Irritates airways and can exacerbate asthma and bronchitis.

• Fine particles are associated with increased risk of cardiovascular events because inhaled particles can trigger systemic inflammation and stress responses.

Reducing these emissions (source-focused concepts)

• Improve combustion efficiency (proper air–fuel mixing, maintenance, avoiding unnecessary idling).

• Reduce fuel evaporation losses (tight storage systems, careful handling).

• Shift to cleaner energy and transportation (reduced reliance on combustion lowers CO, VOC, and PM simultaneously).

• Target high-emitting sources (older vehicles, poorly maintained equipment, high-sulfur or high-ash fuels that increase particle emissions).