What we release into the air

Carbon, methane, particulates, nitrogen oxides and local pollution. Theory, formulas, worked examples and applications.

Not everything that goes into the air does the same thing

Different gases, different mechanisms, different timescales

What comes out of a smokestack or an exhaust pipe is not a single substance, but a mix of very different compounds, each with its own damage mechanism and its own timescale and geographic reach. Carbon dioxide accumulates in the atmosphere for centuries and alters the climate at planetary scale in a practically irreversible way over human timeframes. Methane, by contrast, stays in the atmosphere for much less time — on the order of a decade, against CO2's centuries — but during that time it warms the atmosphere far more intensely per unit of mass, and can escape both from fossil-fuel extraction and transport and from the decomposition of organic waste and certain livestock practices. Suspended particles, especially the smallest ones, penetrate directly into the lungs and can even reach the bloodstream, causing respiratory and cardiovascular damage to those who breathe them, an effect that acts mainly at local scale, near the emission source. Nitrogen oxides contribute to the formation of tropospheric ozone and secondary particles in the atmosphere itself, while sulfur dioxide acidifies soil and water and forms aerosols with their own climate effect. Some of these pollutants act mainly near where they are emitted; others, like CO2, have an effect that is exactly the same regardless of exactly where on the planet they were emitted.

Control systems reduce the damage, but don't eliminate it entirely

An industrial smokestack can incorporate specific filters to capture particles, desulfurisation systems to capture sulfur, and catalysts designed to reduce nitrogen oxides before the gas exits into the atmosphere, along with continuous monitoring systems that verify compliance with established limits in real time. All these systems genuinely reduce the damage caused, but none eliminates it entirely: they in turn consume their own chemical reagents, generate waste that also requires its own management, and none of them automatically captures carbon dioxide, which requires completely different capture technologies still limited in their large-scale deployment. Similarly, an electric vehicle with no exhaust pipe noticeably improves the air quality of the street it drives on, but manufacturing that vehicle, its tire wear, and generating the electricity it consumes all still count in the full environmental balance, even though they are no longer directly visible from the street itself.

"Zero emissions" always needs to say which ones and where

The phrase "zero emissions," used without further precision, almost always hides an implicit analysis boundary worth making explicit before accepting it as a complete statement. A renewable generation plant does not directly emit gases during normal operation, but it did have emissions tied to manufacturing its equipment, a cost that "zero emissions" referring only to the operating phase leaves completely out of the calculation. A plant with carbon-capture technology almost always retains a residual fraction of emissions it fails to fully capture, and also needs additional energy to run the capture system itself, plus transport and geological-storage infrastructure for the CO2 already captured. And offsetting, somewhere else on the planet, emissions produced at a specific location — by buying carbon credits, for instance — does not eliminate at all what was actually emitted at that original location, even though the final global accounting balance may be presented as if it did.

Cutting fossil combustion helps on two fronts at once, but not at the same pace

Reducing fossil-fuel use simultaneously fights global climate change and much of the local air pollution that directly affects the health of nearby populations, because both problems largely share the same physical origin. But that double improvement does not necessarily advance at the same pace in every case: a technology can quickly reduce the most visible local pollution, through relatively accessible filters and catalysts, without having yet proportionally solved the global climate problem, which also requires directly reducing the total volume of carbon dioxide emitted. Measuring the gases and particles actually emitted by a specific source, prioritising protection for the population living closest to that emission source, and evaluating any technology's full lifecycle before simply accepting a "zero emissions" label are the three criteria that allow genuine progress on both fronts at once, instead of solving one while ignoring the other.

Double priority

  • Cutting fossil combustion fights global climate change and much of local pollution at once, but not always at the same pace: don't assume solving one automatically solves the other.
  • Measure the gases and particles actually emitted by a specific source, not a generic "clean" or "dirty" label.
  • Prioritise protection for the population living closest to the emission source: local harm doesn't wait for the global climate balance.
  • Evaluate the full lifecycle before accepting "zero emissions": always ask whether it refers only to operation or to the whole manufacturing, use and end-of-life cycle.

Offsetting elsewhere doesn't erase what was emitted here

Buying carbon credits somewhere else on the planet does not eliminate the particles or nitrogen oxides a specific community breathes near a real emission source. The global accounting balance and the local harm are two different questions that don't substitute for each other.

The numbers, with their source

Methane warms up to 80 times more than CO2 over 20 years, but lasts a tenth as long

According to the IPCC, methane has a global warming potential of between 81 and 87 times that of CO2 over a 20-year horizon (GWP20), but only 28 to 36 times over a 100-year horizon (GWP100) — the difference is because methane stays in the atmosphere for only about a decade, against the centuries or millennia that CO2 emitted today persists. This difference in timescales explains why cutting methane emissions has a relatively fast cooling effect on the climate, while cutting CO2 emissions is the only lever that really counts over the very long term.

Sources: EPA — Understanding Global Warming Potentials; IEA — Methane Tracker 2021, Methane and climate change.

Air pollution kills nearly 7 million people a year

Air pollution — outdoor and indoor combined — is linked to nearly 7 million premature deaths a year worldwide, according to the WHO. Outdoor fine-particle (PM2.5) pollution alone caused, according to more recent data, about 4.9 million deaths in 2023. 89% of these deaths are concentrated in low- and middle-income countries, mainly in South-East Asia and the Western Pacific, and non-communicable diseases — chiefly heart disease and stroke — account for 86% of all deaths attributed to air pollution.

Sources: WHO; State of Global Air 2025; Our World in Data — Data review: how many people die from air pollution?.

What this table is missing

No global average for air-pollution mortality replaces measuring real air quality at a specific place and time: exposure varies enormously depending on proximity to emission sources, local weather, and the emission-control policies in force in each region.

Sources