Vehicles without a valid Pollution Under Control Certificate (PUCC) will not be allowed to enter Delhi from November 1 as the authorities tighten measures to tackle worsening air pollution in the National Capital Region (NCR). The restriction will apply even to Bharat Stage VI (BS-VI) compliant vehicles, which will also need an updated certificate to enter the capital.
The Commission for Air Quality Management (CAQM), which coordinates pollution control measures across Delhi-NCR, had earlier directed fuel stations to refuse petrol and diesel to vehicles without valid PUC certificates.
“Strict vigilance and enforcement of PUC norms for vehicles and stop visibly polluting vehicles by impounding and/or levying maximum penalty,” the Delhi Police said in an advisory on Friday.
However, while the certificate is mandatory, questions remain over how effectively the existing testing system identifies vehicles contributing to air pollution.
What does the PUC test actually measure?
The PUC test checks different emissions depending on the type of vehicle. In petrol, CNG and LPG vehicles, a technician inserts a probe into the exhaust pipe while the engine is idling. The analyser measures carbon monoxide (CO) and hydrocarbons (HC), which indicate incomplete fuel combustion and unburnt fuel.
Newer vehicles are also tested at high idle, with the engine running at around 2,500 revolutions per minute (rpm). The test checks carbon monoxide levels and a parameter called lambda, which measures the air-to-fuel ratio.
Diesel vehicles undergo a smoke test. With the engine warmed up, the technician accelerates the vehicle repeatedly while a smoke meter measures how much exhaust smoke blocks a beam of light.
PUC certificates are generally valid for six months, while vehicles meeting BS-IV and BS-VI standards receive certificates valid for 12 months.
Why the test cannot detect all pollutants
The existing testing method does not measure nitrogen oxides (NOx), which contribute to the formation of fine particulate matter and ground-level ozone.
A 2002 World Bank review found that idle testing “does not represent typical engine operating conditions”. Nitrogen oxides are produced at higher combustion temperatures, unlike the relatively low temperatures generated when an engine idles.
A 2022 report by Dutch research organisation TNO also found that none of the high-emitting nitrogen oxide vehicles in its tests would have failed the existing petrol emission checks.
The system has limitations when it comes to detecting fine particles as well. Modern diesel vehicles use particulate filters to trap soot, but a damaged filter may release particles without producing smoke dark enough for conventional testing equipment to detect.
What causes Delhi's winter pollution?
Delhi's winter haze is largely made up of PM2.5, microscopic particles that can penetrate deep into the lungs. These come from several sources, including vehicle exhaust, crop residue burning, coal, construction dust and household fuel.
A study led by IIT Kanpur, along with IIT Delhi and TERI, found that secondary inorganic particles accounted for 32 per cent of Delhi's winter PM2.5 pollution between November 2022 and February 2023. Biomass burning contributed 24 per cent, while vehicles accounted for 17 per cent.
Vehicles also contribute through nitrogen oxide emissions, hydrocarbons, brake dust and tyre wear.
What are the limitations of India's PUC system?
A Comptroller and Auditor General (CAG) audit covering 2015 to 2020 found irregularities in vehicle emission testing in Delhi. Around 108,000 petrol, CNG and LPG vehicles were certified despite exceeding permissible carbon monoxide or hydrocarbon limits. Another 538,000 diesel vehicles received certificates without recorded test values.
The audit also found that 76,865 tests and certifications were completed in just one minute.
Other countries have adopted more detailed testing methods. China tests petrol vehicles on rollers under load and measures nitrogen oxides, while Hong Kong uses roadside sensors to identify high-emitting vehicles.
India continues to rely largely on the existing idle and smoke tests. The Delhi Assembly has asked the transport department to report by January 31, 2027, on action taken on recommendations arising from the CAG audit.
The November 1 restriction makes a valid PUC certificate essential for entering Delhi, but the effectiveness of the testing system in identifying high-polluting vehicles remains an important concern.