The Commission proposal, REACH (Registration Evaluation and Authorization of Chemicals), has drawn fire from the industry amid concerns it would introduce new costs and red tape for firms without offering substantial benefits.
But environmentalists and consumer organisations have defended the proposal, saying EU citizens come into daily contact with products containing over a 100,000 untested substances. EU industry chief Guenter Verheugen said the Commission would consider changes to the draft law, but wanted to wait for a fresh batch of impact assessments to be completed in March.
There are already 40 reports on the impact of REACH, one of the most fiercely debated EU proposals ever.
"We are not taking a dogmatic approach. We are open to the results from the impact studies," Verheugen told a fully-packed hearing in the European Parliament.
"We will be looking at carrying out the necessary changes."
The EU has been testing new chemical substances since 1981, but there is no such obligation for over 100,000 products that were on the market before then, 30,000 of which are produced in quantities of one tonne or more. REACH would plug this legislative loophole by requiring all substances to be screened.
Stavros Dimas, who heads the Commission's environment department, said the planned costs of REACH were manageable and could be further reduced if better testing methods were developed.
Both commissioners stressed that changes to the draft law should not lower the EU's high environmental standards and said they would be looking for the largest possible consensus.
"The Commission is open to consider proposals that can improve cost-efficiency and workability, provided the level of protection of health and environment is not weakened," Dimas said.
A study by the Global Development and Environment Institute of Tufts University in the United States argued that REACH would cost 3.5 billion euros over 11 years, with annual costs of around 0.06 percent of the chemicals industry's turnover.
Britain and Hungary have proposed to limit the scope of the proposal by suggesting that all chemical substances should be tested and registered only once, rather than at each company.
But several speakers at the hearing said the proposal was unworkable since it would require companies to share data that are normally not given to competitors.
Others criticised the complexity of the proposed rules, and the potential burden on small and medium-sized enterprises (SMEs).
"I have spent a year trying to understand REACH and even now I have difficulties in trying to understand some minor details of it.
"Imagine the difficulties for an SME. Would it not be better to devote that time and money to R&D?," Gyula Kortevelyessy, from the Hungarian Chemicals Association, told the hearing.