In December 1921, chemist Thomas Midgley discovered that the introduction of tetraethyl lead (TEL) in gasoline silenced engine knock — a phenomenon triggered by the auto-ignition of fuel when compressed past its ignition temperature. Engine knock can cause a loss of power and even catastrophic engine damage. Over time, scientists also found that lead additives delivered a boost in octane ratings and mitigated wear and tear on valve seats within the engine.

Notwithstanding warnings of the associated health risks, the sale of the first tank of leaded gasoline commenced in February 1923. However, Midgley wasn’t there. He was in bed, suffering from lead poisoning. Despite multiple reports of illness and death attributed to this lethal additive, the Public Health Service permitted leaded gasoline to remain on the market, powering early model vehicles.

Lead is toxic. Even at low levels it can be hazardous to human health and contaminate air, soil, drinking water and food crops. There has been heightened awareness of the health and environmental impacts of lead since the 1970s. In 1974, the United States Environmental Protection Agency (EPA) announced a phase-out of lead content in gasoline. A complete ban on leaded fuel in on-road vehicles occurred on 1 January 1996 courtesy of the Clean Air Act — a comprehensive federal law that regulates air emissions from stationary and mobile sources.

It was not solely health concerns that initiated a move away from leaded gasoline. TEL is also known to clog catalytic converters — devices introduced by automobile manufacturers to achieve emissions standards by transforming pollutants into less harmful by-products. Unleaded gasoline became widely available in the United States in the mid-1970s, providing protection for an increasing number of cars with catalytic converters.

One would naturally assume that, following a ban on lead, the gasoline pervading our petrol tanks is now 100% lead-free. This is not actually the case. Small traces of lead are still permitted in unleaded fuel. Not-for-profit organisation ASTM International is an originator of voluntary consensus standards. The ASTM Organometallic Task Group is responsible for guidance on the appropriate concentration limits for organometallic compounds as set out in ASTM standard D4814.

At the latest Organometallic Task Group meeting, held in Denver, Colorado, U.S.A., on 24 June 2019, the Task Group requested approval to ballot a reduction in the D4814 lead limit for unleaded gasoline. The current limit of 0.013g/L is based on a standard set in 1975 and is no longer aligned with much of the industrialised world. The proposal suggests the ASTM International standard to be aligned with other leading nations. When the original lead limit was instigated, the suggestion was for a substantially lower value of 0.008g/L. The limit was relaxed to allow for residual lead in the pipeline system.

According to Stratas Advisors’ 2018 Global Fuel Specifications Report, many countries have already shifted to 0.005g/L limits for lead — including 28 countries in Europe, Canada, China and Australia.  The U.S.A. sits alongside Bolivia, Colombia, Chile, Saudi Arabia, Iran, Malaysia, Pakistan, South Korea and others who continue to legislate a higher lead limit.

As of March 2017, only Algeria, Yemen and Iraq continue to employ widespread use of leaded motor gasoline, according to a report entitled Leaded Petrol Phase-out: Global Status as at March 2017 by the United Nations Environment Program (UNEP) – Partnership for Clean Fuels and Vehicles.

In the proposal to ballot, the Organometallic Task Group highlighted that by 2025, light-duty vehicles must meet super ultra-low emissions vehicle 30 (SULEV30) emission levels, on average, requiring a lead limit in Tier-3 and LEV-III certification fuel of 0.0026 g/L. This is the same lead limit as required in ASTM specifications D5059 and D3237.

The proposal also cited unpublished OEM data indicating an aftertreatment system that was “quickly deactivated when a vehicle was fuelled with gasoline containing 0.013g/L Pb (lead).” Though, the application also acknowledged there are no recent known instances of lead contamination in U.S. gasoline.

Changes to the ASTM D4814 standard has implications for several other countries. Egypt, Colombia, Jamaica, Ghana, El Salvador, Nicaragua, Panama, Afghanistan, Peru and Uruguay have indicated the use of  D4814 as the basis for their own national standard; whereas Ethiopia, the Philippines and Zambia have indicated a degree of adoption of D4814. Several other nations either directly reference D4814 in their regulations or have consulted ASTM International during the development of their national standards.

Prior to the proposal to ballot changes to D4814, a Co-ordinating Research Council (CRC) project (RW-108), intended to ascertain the appropriate limit for lead in gasoline for light-duty vehicles to achieve Tier-3/LEV-III emission standards, was proposed. The task group acknowledged receipt of two “technically-viable” proposals, however, both proposals were rejected due to exceeding the funding available to run a test program.