Investment funds that market themselves as sustainable under European rules -- otherwise known as environmental, social and governance (ESG) investing -- get similar levels of "green revenues" through the companies they invest in as traditional funds, according to a new study published on Thursday.

So-called "green revenue" is the percentage of sales generated from products that benefit the environment such as with cleaner water or contribute to mitigating climate change.

Clarity AI, a sustainability data technology platform part-owned by BlackRock, analyzed 31,000 funds to assess how they perform against a new European Union classification system that defines a list of environmentally sustainable economic activities. Its research found that just 3.6% of revenues globally are "green."

Under the European Union's Sustainable Finance Disclosure Regulation, fund managers can classify their products by the degree to which they promote environmental, social and governance goals.

They can market themselves as either Article 8, which means fully or partly focused on environmental, social or sustainability issues, or a more stringent Article 9, which means fully focused on sustainable objectives.

Investments that fund firms class as Article 6 mean that they do not focus on sustainability.

Clarity found that for Article 8 funds, 3.9% of their revenue through the portfolio companies they own can be classified as green, similar to the 3.1% share for Article 6 products.

Funds in the smaller pool of Article 9 had 15% of revenue as green.

"Article 8 funds are defined as 'promoting E or S characteristics', which is a vague term. As such, we are seeing the market interpreting this term often by integrating a very light touch of sustainability-related considerations into their strategy," Patricia Pina, Clarity AI's Head of Product Research & Innovation, told Reuters.

While new and still-emerging EU regulatory frameworks have been hailed for trying to improve transparency in the fast-growing world of ESG finance, vague definitions can result in managers loosely interpreting the sustainability credentials of their funds.

Earlier this year Morningstar, which closely tracks the funds industry, stripped more than 1,000 funds of their ESG label in its classification system after finding they did not live up to their sustainability claims.