A cement company has been fined £99,000 after admitting nine offences, including releasing more than the permitted amount of dioxins.
Castle Cement has invested £72m in its new factory
Flintshire magistrates also told Castle Cement to pay £9,300 costs over its plant at Padeswood, near Mold.
The Environment Agency brought the case over old kilns, which are no longer in use after investment in new ones.
The firm said there were problems with monitoring, but it was not convinced that the emission figures were correct.
The offences covered March 2004 to January 2005, and the company asked for 29 other matters to be taken into consideration.
Neil Evans, prosecuting, said the breaches were not deliberate, but a failure to notify the agency "must have been the result of a deliberate decision by somebody within the company".
"Overall, the offences indicate a significant failure to achieve the high standard of care required, bordering on recklessness," he said.
Mr Evans said there was a potential for harm, although reports on the impact on health said the release was "not considered serious".
But there was a "considerable mass releases of dioxins compared with the historical performance at Padeswood and the highest mass emissions from any site regulated by the Environment Agency," he added.
Brian Greenwood, defending, said that it was not a cowboy outfit or a rogue operation but an international company which had employed international consultants to carry out the monitoring.
Mr Greenwood said the company suspected the readings were wrong at the time but there was no suggestion that they were above the limit.
As a comparison, he said if the Millennium Stadium was full of golf balls, the dioxins would be one of them.
The company, which had a turnover of £191m, and pre-tax profits of £13m, had invested £72m in a new kiln which was "cutting-edge technology".
The company admitted three charges that its emissions exceeded dioxins limits, three that it failed to notify Environment Agency Wales promptly, and three of failing to properly monitor, or take representative samples, from the kiln stacks.
Magistrates' chairman Edwin Hughes said he was very concerned that the company ran its operation for nine months without knowing dioxin levels and emissions.
Castle Cement later blamed delays in approving plans for the new kiln for the emissions problem.
Speaking after the court hearing, Danny Coulston, general manager of the Padeswood works, said: "I am disappointed and frustrated that we have been prosecuted over dioxin emissions from our older, now shut down kilns.
"Castle Cement was unable to replace these kilns as quickly as the company would have wished because of severe delays caused by the planning inquiry for the new kiln four."
Mr Coulston said the firm had been forced to continue operating "less efficient kilns for far longer than we wanted".
He added: "Since our new kiln came into operation, dioxin levels have been reduced to levels comparable with the best in the world, sulphur dioxide levels by 95%, carbon dioxide levels by about 20%".