• CO2 is not an issue regarding emissions reduction. It is best addressed via fuel consumption reduction.
• CO should not pose a problem - except in confined spaces. Although being toxic, It is comparatively easily oxidized to CO2.
• NOx is an issue to be addressed as it is rather stable and forming all these nitric acids when humidity is available.
• HC is another issue: this is not a single compound but a wild mixture of hydrocarbons, some of them toxic and long-lasting.

HC and CO tend to decrease with an increase of combustion efficiency while NOx tends to increase under the very same conditions. HC and CO are relatively easy to address with an oxidizing catalytic converter.

NOx poses some problem: it increases with improvements of the combustion process (which typically results in the reduction of HC and CO), and is comparatively difficult to tread - requiring the so-called 3-way catalytic converter.

Carbon monoxide (CO) probably is the most most harmful of these gases (assuming the gases are compared at equal concentrations). CO binds more tightly than O2 with hemoglobin and thus causes oxygen deprivation.

• CO2 is not an issue regarding emissions reduction. It is best addressed via fuel consumption reduction.
• CO should not pose a problem - except in confined spaces. Although being toxic, It is comparatively easily oxidized to CO2.
• NOx is an issue to be addressed as it is rather stable and forming all these nitric acids when humidity is available.
• HC is another issue: this is not a single compound but a wild mixture of hydrocarbons, some of them toxic and long-lasting.

HC and CO tend to decrease with an increase of combustion efficiency while NOx tends to increase under the very same conditions. HC and CO are relatively easy to address with an oxidizing catalytic converter.

NOx poses some problem: it increases with improvements of the combustion process (which typically results in the reduction of HC and CO), and is comparatively difficult to tread - requiring the so-called 3-way catalytic converter.

Dear Farzin Zamankhan,

The sequence of the  emission gases from a gasoline combustion engine are arranged as following:

1) From an environmental point of view (Harmful on Greenhouse Gases Emissions):the sequence will be  CO2 then, CO and NOx

2) From Human Health and Safety point of view: the sequence will be  CO then, CO2 and NOx

Known that another harmful compound also my be present from gasoline combustion, but in limited amounts such as SOx, and particulate matter. 

Best regards

Much of harmful gases are produced either naturally or by various human activities. Brief description and effects of six important greenhouse gases are given below:

a. Carbon dioxide (CO2) – A naturally occurring gas produced by living organisms and fermentation. It is a normal component of the breath we exhale; it is hazardous in concentrated volumes. Large quantity of carbon dioxide is produced by the combustion of carbonaceous fuels. Carbon dioxide emissions from fuel burning, responsible for about 87 percent of global warming, have increased by about 27 percent since the industrial revolution.

b. Nitrogen oxides (NOx) – Nitrogen oxides are naturally occurring from microbial action in soil. NOx is also produced by fuel burning. Scientists say its production is increased by the use of nitrogen based fertilizers in agriculture, as well as by the use of catalytic converters in automobiles.

c. Methane (CH4) – Methane is a naturally occurring, inflammable gas. Methane is produced by geological coal formations and by the decomposition of organic matters. Leading man-related sources of methane are landfills; livestock digestive processes and waste, especially ruminants (cud-chewing animals); and wetland rice cultivation.

d. Hydrofluorocarbon gasses (HFCs) – Chlorofluorocarbons (CFC), the coolant, cleaning, and propellant gases were blacklisted internationally due to its ozone-eating characteristics. HFCs do contribute to global warming. Global warming potential of HFCs is 4,000 to 10,000 times that of CO2.

e. Perfluorocarbons (PFC), or perfluoro compounds – Man-made replacement gases for CFCs but the result also as a by-product of aluminium smelting. PFCs also used as a purging agent for semiconductor manufacture and small amounts are produced during uranium enrichment processes. Global warming potential of Perfluorocarbons (PFC) or perfluoro compounds is 6,000 to 10,000 that of CO2.

f. Sulfur hexafluoride (SF6) – Very low atmospheric concentration makes it an ideal test gas for gas concentration monitors. Principal uses the insulating material for high-voltage equipment like circuit breakers at utilities. Also used in water leak detection for cable cooling systems. SF6 is a man-made gas.

g.Carbon monoxide (CO) is a colourless, odourless, and tasteless gas that is slightly less dense than air. It is toxic to hemoglobic animals (both invertebrate and vertebrate, including humans) when encountered in concentrations above about 35 ppm, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal biological functions. In the atmosphere, it is spatially variable and short lived, having a role in the formation of ground-level ozone.

This information can help you to make the list. However, you have to take into account, the concentration of the substance, the place/space where the gas is detected or used, among others.