Dozens of workers were rushed to hospital after inhaling ammonia gas at a meatpacking plant in Maraba, Brazil on Monday.

At least 42 employees of the JBS slaughterhouse fainted when they breathed in the hazardous gas used as a refrigerant on June 12, authorities said.

The workers had fled the plant after the first 'weak' leak at 9:30 am but returned after the situation was brought under control. However, a second stronger leak occurred an hour later, causing them to become dizzy and faint.

They were taken to the Municipal Hospital of Maraba, where 36 of the 42 people have been stabilised, local media reported.

One employee told local reporters that she had only been able to save herself by holding her breath until she reached a safe distance. She added that this was not the first time such an incident had happened at her workplace.

JBS said in a statement: 'The situation at the Maraba unit is back to normal. Employees who required medical assistance were promptly attended to, and all presented mild symptoms.'

The Para Civil Police are consulting with experts to investigate the accident. No fatalities have been reported.

Ammonia is a colorless, pungent, highly-soluble gas mainly used in the manufacture of fertilizers, nitric acid, and other nitrogenous compounds, and as a refrigerant and solvent. Very high levels of ammonia can damage the lungs or cause death.