Non-methane hydrocarbons (NMHC): what they are, sources and environmental impact

Non-methane hydrocarbons (NMHCs) are chemical substances that remain in a gaseous state at room temperature and belong to the group of volatile organic compounds (VOCs). Within this large family of compounds, NMHCs are among the most chemically reactive and take part in photochemical reactions, contributing to the pool of reactive carbon in the atmosphere. They are composed solely of carbon and hydrogen atoms and originate mainly from human activities. Their environmental relevance lies in their role as precursors of tropospheric ozone and other oxidising agents, making them a major contributor to air pollution.

Due to their behaviour as secondary pollutants and their potential carcinogenic effects, NMHCs are considered key indicators in air qualityAir quality refers to the state of the air we breathe and its composition in terms of pollutants present in the atmosphere. It is considered good when poll...
Read more management. For this reason, both European and Spanish legislation regulate their presence through directives, laws and royal decrees to establish surveillance and control systems based on scientific evidence of their environmental toxicity and effects on human health.

What are non-methane hydrocarbons (NMHCs)?

Non-methane hydrocarbons form an essential fraction of volatile organic compounds found in the atmosphere. Their environmental importance derives from both their abundance in the air and the role they play in photochemical processes that produce secondary pollutants. Although their composition and concentration vary depending on the environment and emission sources, studying them is crucial to understand tropospheric ozone dynamics and to monitor urban air pollution.

Technical and chemical definition of NMHCs

From a chemical perspective, NMHCs are hydrocarbons or molecules composed exclusively of carbon and hydrogen atoms, like methane (CH₄)Methane, known chemically as CH₄, is a gas that is harmful to the atmosphere and to living beings because it has a high heat-trapping capacity. For this ...
Read more, but they differ from methane because they range from simple alkanes to complex aromatic structures similar to benzene rings. While methane shows limited atmospheric reactivity and a long lifetime, NMHCs have high reactivity due to their interaction with oxidising radicals such as the hydroxyl radical (·OH), making them key participants in photochemical reactions near the Earth’s surface.

The most significant NMHCs in the atmosphere include ethane, ethylene, propane, toluene, xylene and benzene, the latter being highly toxic and carcinogenic. These compounds originate mainly from industrial processes, combustion emissions, organic solvents, road traffic and oil refining operations.

NMHCs vs VOCs: differences and classification

Broadly speaking, VOCs can be classified according to their chemical structure (aliphatic, aromatic, oxygenated, halogenated, etc.) and their photochemical reactivity. NMHCs, as the most reactive and potentially harmful subset of VOCs, are under special surveillance in air quality networks due to their role in the formation of secondary pollutants, tropospheric ozone and oxidants, as well as their contribution to public health risks.

Main category	Subtype / Family	Typical examples (formula)	Key characteristics	Predominant sources
Saturated (paraffins)	Linear and branched alkanes	Ethane, propane, n-butane, isobutane, n-pentane, n-hexane, n-heptane (CnH2n+2)	Low to medium photochemical reactivity with OH radicals, but their high concentration contributes to ozone and secondary organic aerosol (SOA) formation.	Fossil fuels (natural gas, LPG, petrol, diesel), fuel evaporation, storage leaks, vehicle exhaust and combustion equipment.
Unsaturated (olefins)	Alkenes	Ethene (ethylene), propene, butenes (CnH2n)	Highly reactive with OH radicals and ozone; key in photochemical smogSmog, beyond that dense fog Smog is a mixture of air pollutantsAir pollution caused by atmospheric contaminants is one of the most critical and complex environmental problems we face today, both because of its global r... Read more that accumulate in the atmosphere, especially in urban areas. This phenomenon is character... Read more and tropospheric ozone production.	Vehicle exhaust (especially petrol), incomplete combustion, refineries, petrochemical industry, cracking processes.
Unsaturated (acetylenic)	Alkynes	Acetylene (C2H2)	Moderate to high reactivity; used as a tracer for combustion and traffic emissions.	Incomplete combustion in engines, high-temperature industrial processes, welding.
Aromatics	Monoaromatics	Benzene, toluene, ethylbenzene, xylenes (BTEX)	High ozone and SOA-forming potential; several are toxic and/or carcinogenic (e.g. benzene).	Petrol and solvents, traffic emissions (evaporative and exhaust), paint and chemical industries.
Aromatics	Polycyclic (PAHs with only C and H)	Polycyclic aromatic hydrocarbons without heteroatoms	Less volatile, partly particle-bound; contribute to SOA and are highly toxic, classified as PAHs.	Coal, biomass and petroleum combustion, metallurgical processes.
Cycloalkanes	Cycloparaffins	Cyclopentane, cyclohexane and homologues	Intermediate reactivity; contribute to ozone and SOA; less abundant than linear alkanes in urban air.	Fuels and solvents, petrochemical processes and refineries.
Non-methane biogenic	Isoprene and hydrocarbon terpenes	Isoprene, monoterpenes (e.g. α-pinene, limonene) without oxygen in their structure	Extremely reactive with OH radicals and ozone; significant ozone and SOA contributors, especially in vegetated areas with high NOx levels.	Emissions from vegetation (forests, crops, urban green areas); biogenic but influencing anthropogenic photochemistry.
Regulated mixtures	Total NMHCs (operational sum)	Sum of all hydrocarbons except methane	Aggregated indicator of ozone and smog precursors; measured in inventories and regulations (e.g. exhaust or stack emissions).	Fuel trade and transport, combustion engines, industrial sources and urban VOC mixtures.

Environmental and health impact of NMHCs

Although not widely known, NMHCs play a key role in the development of atmospheric pollution in both urban and industrial environments. Their presence in the air not only contributes to the formation of tropospheric ozone and secondary particles, but also poses a threat to human health due to their toxicity and long-term effects. Assessing their environmental and health impact helps to better understand the need for strict emission control through continuous environmental monitoring to enable a sustainable approach to air quality management.

Formation of tropospheric ozone and photochemical smog

Once released into the atmosphere, NMHCs undergo a series of chemical reactions that determine their persistence and environmental impact. The most relevant reaction is their oxidation in the presence of hydroxyl radicals (·OH), ozone (O₃) or nitrate radicals (NO₃), leading to the formation of aldehydes, organic peroxides and other highly reactive intermediate compounds. These reactions, combined with nitrogen oxides (NO_x), indirectly produce tropospheric ozone and other photochemical oxidants, contributing to the occurrence of harmful smog episodes that are common in urban and industrial environments.

From an atmospheric dynamics perspective, NMHCs have atmospheric lifetimes ranging from a few hours to several days, depending on their chemical structure and environmental conditions. Light, volatile and reactive compounds oxidise quickly, whereas heavier aromatics may persist longer or settle on surfaces, affecting both ecosystems and materials.

This chain of transformations makes NMHCs crucial intermediates in the reactive atmospheric carbon cycle and central components of the chemical processes that define air quality, which is why they require strict environmental control.

Effects on human health

Exposure to NMHCs poses a significant risk to human health, particularly in environments where their concentration exceeds established safety levels. Due to their reactive nature and their ability to be absorbed through the respiratory tract, NMHCs can cause acute and chronic respiratory disorders. Irritation of the mucous membranes, coughing, shortness of breath and the aggravation of pre-existing pulmonary conditions such as asthma or bronchitis are among the most immediate and harmful effects of exposure to elevated concentrations of NMHCs in the air.

Certain aromatic hydrocarbons, such as benzene, toluene and xylenes, also present a recognised carcinogenic and neurotoxic potential. Benzene, in particular, is classified as a human carcinogen by international organisations and is linked to the development of leukaemia and other haematological disorders. Toluene and xylenes, though less potent, have effects on the central nervous system, causing headaches, fatigue and loss of coordination after prolonged exposure.

Benzene, one of the most hazardous BTEX (benzene, toluene, ethylbenzene, xylene) pollutants, has been classified as a Group 1 human carcinogen by both the USEPA and IARC. Long-term exposure to benzene is associated with the development of leukaemia and other haematological cancers, as well as bone marrow damage and immune system impairment. Exposure to toluene and xylene is linked to neurotoxic effects such as headaches, fatigue, tremors and loss of concentration, with chronic exposure potentially leading to severe neurological disorders. Kumari, P. et al. Environmental Analysis Health and Toxicology, 2024.

In the long term, continuous exposure to subcritical concentrations of NMHCs may contribute to systemic dysfunctions, endocrine disruption and bioaccumulative effects in fatty tissues. Due to their toxicity and interactions with other air pollutants, NMHCs represent a complex vector of health risk that requires not only emission control but also active epidemiological surveillance linked to air quality monitoringControlling air quality is an essential task in order to enjoy optimal environmental conditions for healthy human development and to keep the environment i...
Read more.

Ecological and climatic effects

NMHCs play an important role in oxidative atmospheric processes that disrupt the balance of greenhouse gasesWhile the concentration of carbon dioxide (CO2) in the atmosphere has been steadily and rapidly increasing in recent decades, in May 2025, CO2 surpassed 43...
Read more. Their chemical interaction with radicals and other pollutants contributes to the formation and accumulation of tropospheric ozone, a gas with local warming potential. Additionally, some NMHCs indirectly influence the carbon cycle and other atmospheric processes that affect the Earth’s radiative balance, ultimately impacting climate dynamics on both regional and global scales.

In ecosystems, these chemical transformations lead to the production of secondary compounds that degrade air quality, damage vegetation and alter ecosystem composition. Therefore, NMHCs contribute to both environmental degradation and climate warming, highlighting the need to control and monitor their emissions to mitigate their adverse effects on the environment and climate.

The Port of Santander stands out for reducing non-methane hydrocarbon and other pollutant emissions by electrifying its docks, allowing ships to shut down their auxiliary engines while in port. This measure helps to improve air quality and prevent these compounds from harming aquatic ecosystems and biodiversity. The electrification process relies on renewable energy, ensuring a cleaner power supply and reducing environmental impacts on air and water quality in port areas.

Measurement and monitoring of non-methane hydrocarbons

Measuring and monitoring non-methane hydrocarbons is essential to assess and control air quality in urban and industrial environments. Accurate quantification is crucial since these volatile organic compounds, apart from methane, play a major role in the formation of tropospheric ozone and other secondary pollutants.

Analytical methods and current regulations

The analysis of non-methane hydrocarbons in ambient air relies on precise, standardised techniques based on automated sampling and instrumental analysis to achieve both characterisation and quantification.

One of the most widely used instrumental methods is flame ionisation detection (FID), which measures the total concentration of NMHCs, excluding methane, with high sensitivity. For more detailed qualitative and quantitative information, gas chromatography (GC) coupled with detectors such as FID (GC-FID) or mass spectrometry is used. These techniques allow the individual separation and analysis of volatile organic compounds present in air samples.

Sampling through automated monitoring systems operating continuously enables the identification of NMHC emission sources, monitoring of temporal variations and verification of regulatory compliance.

Regarding applicable regulations, key international and European standards include EPA/600, EN 13526, CEN/TS 17660-1/-2 and UNE-EN 14662, which establish procedures, technical requirements and quality criteria to ensure data comparability. These regulations are essential for effective environmental management, ensuring emission reduction and protecting public health.

Instrumental techniques and sensors

Instrumental characterisation of NMHCs has significantly evolved thanks to advances in gas chromatography and the development of smart sensors—innovations that mark a new era in environmental monitoring and NMHC management.

The GC-FID technique (gas chromatography with flame ionisation detection) remains the reference standard due to its high sensitivity and selectivity for detecting the non-methane fraction, allowing the identification and quantification of numerous compounds in specialised laboratories. Meanwhile, photoionisation detectors (PID) offer speed and versatility in the field, enabling rapid detection of NMHCs and other volatile organic compounds in industrial or urban areas.

The deployment of portable sensors based on photoionisation and advanced chromatography technologies has enhanced the flexibility and efficiency of on-site monitoring, providing valuable data for environmental decision-making and pollution control.

In this context, smart sensors such as those developed by Kunak stand out for their ability to directly measure specific NMHC fractions, using interchangeable cartridges, automated calibration and digital data reporting via cloud platforms. This advanced technological approach enhances traceability, reduces operational costs and allows integration into multipoint networks for comprehensive environmental surveillance.

The incorporation of wireless connectivity and solar power supply ensures autonomous operation even in remote areas, consolidating these systems as essential tools to achieve regulatory compliance, environmental protection and improved air quality in both urban and industrial settings.

Real-time monitoring solutions

Specifically, Kunak AIR Pro stations exemplify the technological progress achieved in environmental monitoring systems by combining metrological precision, advanced connectivity and user-friendly operation in a single device. This professional monitor is equipped with a dedicated smart cartridge for NMHC detection, allowing for quick and easy replacement without the need for laboratory servicing, while ensuring automatic calibration and certified traceability to guarantee data reliability.

The system provides continuous monitoring and real-time recording of ambient NMHC levels, adapting to industrial, urban and scientific environments thanks to its high capacity to measure gases and particles simultaneously. Equipped with IoT connectivity, it transmits data in real time to Kunak AIR Cloud digital platforms, enabling data analysis, early alerts and automatic report generation.

These features make Kunak AIR Pro a benchmark in professional NMHC monitoring, ensuring full regulatory compliance while optimising air quality management and control processes. The integration of smart technologies and solar panels for autonomous operation allows for scalable and efficient deployment, even in the most remote locations.

Regulations and concentration limits

The recognition of the environmental and health impact of NMHCs has led to the implementation of increasingly strict regulatory frameworks at the international level. These frameworks include standards and guideline values that guide the measurement, control and reduction of emissions in both industrial and urban environments. Regulation is based on scientific evidence that directly links NMHC concentrations and secondary compounds to adverse effects on air quality, public health and climate balance. Consequently, European institutions, American agencies and global organisations have established specific guidelines to monitor and limit the presence of these compounds in the atmosphere, aligning regulatory efforts with technological progress and the need for real-time environmental monitoring.

European legislation, EPA and WHO

In Europe, the Directive 2008/50/EC on ambient air quality establishes limits and target values for related compounds, such as total hydrocarbons and ozone, which include NMHC fractions.

The United States Environmental Protection Agency (EPA), through the Clean Air Act, defines air quality standards for total volatile organic compounds (TVOC) and tropospheric ozone, recognising the precursor role of NMHCs in their formation and toxicity.

The World Health Organization (WHO), in the latest version of its Air Quality Guidelines, establishes exposure values for ozone, reflecting its impact on human health and the environment. Although it does not include specific numerical guidelines for VOCs or TVOCs, the WHO highlights the need to control NMHCs to protect public health and mitigate photochemical pollution events in the atmosphere.

Reduction strategies and public policies

Strategic responses to the NMHC problem are based on establishing accurate emission inventories and comprehensive air quality plans that identify both point and diffuse sources.

Public policies promote industrial emission controls, reduction of leaks in production processes and urban management strategies aimed at minimising mobile sources and activities with a high NMHC emission potential. Controlling diffuse emissions, linked to solvent and fuel volatilisation, is key to reducing total NMHC loads in the environment.

Consequently, the development of clean technologies, promotion of renewable energies and improvements in waste management complement these NMHC reduction measures. The combined implementation of these policies, supported by continuous monitoring and evaluation, represents the most effective path toward sustainable air quality improvement and reduction of NMHC impact at both regional and global levels.

Relevance of NMHCs in urban air quality

NMHCs are critical components in assessing and managing air quality in urban environments, where the complex mix of anthropogenic and natural sources generates significant impacts on public health and the environment. Their presence and concentration not only reflect the magnitude of atmospheric pollution but also help anticipate the formation of secondary pollutants such as tropospheric ozone and fine particles. Therefore, NMHC analysis has become an essential tool in integrated environmental monitoring systems and in shaping urban and industrial policies aimed at reducing pollution levels and improving quality of life.

NMHCs as indicators of pollution

NMHCs act as sensitive indicators of urban air pollution due to their high chemical reactivity and predominantly anthropogenic origin, linked to road traffic, industrial activities and energy use. Their concentration correlates directly with the Air Quality Index (AQI/ICA), which integrates multiple parameters to reflect the health risks associated with exposure to NMHCs. Thus, the continuous and accurate measurement of NMHCs supports early warning systems and assessment of mitigation effectiveness, providing a key indicator for informed environmental decision-making.

The importance of NMHC monitoring campaigns

Several urban and rural studies have demonstrated the crucial role of NMHCs in characterising atmospheric pollution. In densely populated cities, monitoring stations equipped with advanced sensor technology, such as those provided by Kunak, have collected data revealing the dynamics of these compounds in relation to other critical pollutants such as nitrogen oxides (NO_x) and ozone (O₃).

These campaigns show a clear correlation between NMHC peaks and episodes of photochemical ozone formation, particularly during periods of high solar radiation. Moreover, sensor network data allow for the identification of seasonal and spatial patterns, supporting the development of local, evidence-based strategies for continuous air quality improvement.

How NMHCs contribute to modelling and environmental health

The integration of NMHCs into advanced dispersion and atmospheric chemistry models is essential for understanding their dynamics and impact on air quality and public health. NMHCs not only act as precursors of secondary pollutants such as tropospheric ozone, but their inclusion in models also helps to improve the accuracy of predictions for concentration and exposure under various atmospheric conditions. These models, combining physical, chemical and meteorological processes, serve as essential tools for anticipating pollution episodes, assessing health risks and designing more effective environmental policies.

Integration into dispersion and atmospheric chemistry models

Computational models such as CMAQ (Community Multiscale Air Quality Model), WRF-Chem (Weather Research and Forecasting with Chemistry) and CAMx (Comprehensive Air Quality Model with Extensions) incorporate the chemical and physical properties of NMHCs to simulate their atmospheric behaviour.

These models also consider meteorological variables, emissions, chemical reactions and transport processes to accurately represent the dispersion, transformation and fate of NMHCs in the atmosphere. Accurate representation of these compounds is crucial due to their role in ozone and fine particulate formation, as well as their varying persistence depending on chemical structure. Their detailed representation makes CMAQ, WRF-Chem and CAMx leading tools for managing air pollution in urban and industrial environments.

Applications in public health policy and early warning systems

Computational models that include NMHCs enable the design of more accurate early warning systems for potential pollution episodes, facilitating preventive measures and protection of vulnerable populations. Simulations based on these models are used by regulatory agencies to evaluate potential emission impacts, optimise air quality plans and support policy-making to protect public health.

Furthermore, the detailed prediction of ozone and other photochemical oxidant levels — of which NMHCs are precursors — supports sustainable urban planning and helps reduce the incidence of respiratory and cardiovascular diseases associated with air pollution.

Frequently asked questions about non-methane hydrocarbons (NMHCs)

What is the difference between NMHCs and VOCs?

NMHCs are a specific category within VOCs that excludes methane due to its low atmospheric reactivity. Although both groups are relevant in atmospheric chemistry, NMHCs have a more direct impact due to their high reactivity, contributing to the formation of secondary pollutants such as tropospheric ozone and other air oxidants that degrade air quality.

How are non-methane hydrocarbons measured in the air?

Measuring NMHCs is essential as they significantly contribute to tropospheric ozone formation and other secondary pollutants, affecting air quality and public health in both urban and industrial environments. The most common measurement methods are:

	Automated sensor monitoring	Flame ionisation detection (FID)	Gas chromatography (GC)
Principle	Smart sensors with autonomous technology, automatic calibration and cloud data transmission.	Measures total NMHC concentration excluding methane using flame ionisation.	Individual identification and quantification of compounds through chromatographic separation and detection (FID or mass spectrometry).
Accuracy	Lower accuracy at low concentrations; limited sensitivity and selectivity.	High sensitivity but less selectivity for specific compounds.	High precision and selectivity for individual components.
Continuity and real time	Continuous and automatic measurement with operational efficiency and traceability.	Fast response; allows rapid readings but requires frequent maintenance.	Not continuous or real time; long analysis cycles (15–60 min).
Interference sensitivity	Affected by temperature, humidity and pressure; may impact stability and cause signal drift.	Sensitive to gas interferences and environmental conditions such as humidity.	Sensitive to interferences; requires pure gases and controlled conditions.
Maintenance and calibration	Requires frequent calibration for reliability; less robust technology.	Needs regular maintenance and calibration.	Complex calibration and maintenance; requires specialised personnel.
Costs	More economical; solar power reduces operating costs and allows dense deployment.	High cost; uses combustible gases and delicate components.	Very high operational and infrastructure cost.
Data and applications	Less consistent data, useful for identifying trends and issuing general alerts.	Reliable data on total NMHC quantity with high speed.	Detailed and specific data suitable for scientific studies and regulatory compliance.

What are the main sources of NMHCs?

The anthropogenic sources of NMHCs encompass a wide range of human activities. Among the most relevant industrial sources are:

• Road transport
• Fuel evaporation
• Refining and storage of hydrocarbons
• Industrial activities using organic solvents

In urban environments, the main source of NMHC emissions is road traffic, which releases aromatic and paraffinic compounds in varying proportions depending on fuel type and engine efficiency.

Why are NMHCs an environmental problem?

NMHCs are an essential factor in urban and industrial air pollution because they react in the atmosphere with hydroxyl radicals (·OH), ozone (O₃) and nitrate radicals (NO₃), generating reactive intermediates such as aldehydes and peroxides. These reactions, together with nitrogen oxides (NO_x), form tropospheric ozone and photochemical oxidants responsible for urban and industrial smog, which alter existing greenhouse gases and affect regional and global climate dynamics.

NMHCs can remain in the air for hours or days, depending on their chemical structure and environmental conditions. While lighter compounds oxidise quickly, heavier aromatic compounds may persist longer and eventually affect ecosystems and materials.

Their toxic nature and long-term health risks have been scientifically proven, making strict environmental control essential through continuous monitoring to track their presence as central agents in the reactive carbon cycle of the atmosphere, ensuring sustainable air management.

What solutions does Kunak offer to measure NMHCs?

Kunak AIR Pro is an advanced environmental monitoring station that combines metrological precision, IoT connectivity and ease of use in a single professional device. It is specifically designed for the detection of non-methane hydrocarbons (NMHCs) using interchangeable smart cartridges, which enable automatic calibration without the need for laboratory intervention. This is a comprehensive solution for professional NMHC monitoring, combining smart technology, operational efficiency and scientific accuracy to improve air quality management and support environmental sustainability.

Its main technological advantages include:

• Simultaneous measurement of gases and particles in real time.
• Continuous monitoring adaptable to urban, industrial and scientific environments.
• Data transmission to digital platforms (Kunak AIR Cloud) for analysis, early alerts and automatic reporting.
• Certified traceability and high data reliability.

In addition, Kunak AIR Pro offers remarkable operational benefits:

• Quick cartridge replacement with no downtime.
• Autonomous operation powered by solar panels.
• Scalable deployment even in remote locations.
• Optimised regulatory compliance for environmental standards.

Driving NMHC environmental management through innovation and precision

Non-methane hydrocarbons (NMHCs) have become critical compounds for diagnosing and controlling air quality due to their direct role in photochemical processes that generate tropospheric ozone and secondary particulate matter. Their influence on human health and the environment demands rigorous and continuous monitoring, aligned with increasingly strict national and international regulatory standards, as well as the implementation of effective strategies for air pollution control and mitigation.

In this global context, the adoption of cutting-edge technologies such as Kunak’s smart sensors represents a decisive step forward in controlling these compounds. These solutions enable precise, traceable and automated monitoring of NMHCs, integrating remote calibration capabilities and generating scientifically validated data. This approach allows for a robust, transparent and adaptive environmental management system capable of meeting the challenges of atmospheric pollution.

Environmental professionals, urban and industrial technical managers and regulatory authorities need to integrate these technological tools to strengthen atmospheric monitoring, enhance public health protection and ensure regulatory compliance in a global context increasingly committed to environmental and social responsibility.