The kilogram is one of seven International System of Units (SI) of measure and is defined by the International Prototype kilogram (IPK) which is a right-circular cylinder made of 90 percent platinum and 10 percent iridium that is stored at the Bureau International des Poids et Mesures in Sèvres, France.

40 replicas were made in 1884 and sent around the world in an effort to standardize the measure of mass. Scientists at Newcastle University in the UK now said that UK's replica number 18, which is stored at the National Physical Laboratory (NPL) has changed its mass since 1884 by "tens of micrograms" due to the accumulation of hydrocarbons.

Research lead Peter Cumpson, professor of Micro Electro Mechanical Systems at Newcastle University, said that "mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge." He noted that "there are cases of international trade in high-value materials - or waste - where every last microgram must be accounted for."

Cumpson said he used an x-ray photoelectron spectroscopy machine (XPS) to analyze the IPK's surface and found that a "suntan" could remove the excess weight again. According to the scientist, the "carbonaceous contamination" could be correct by exposing the surface to "a mixture of UV and ozone", which would not damage the platinum surface of the cylinder.

Newcastle University said that "work is underway internationally in several National Measurement Institutes to find an alternative to the IPK – a standardised value for the kilogram that is not based on a matchbox- sized piece of metal." However, until such a replacement is found, scientists have to make sure that the kilogram remains a kilogram: "If the kilogram does put on weight then it's imperative that we understand exactly how the IPK is changing," Cumpson said.