Washington, Sep 1 - Exposure to carbon monoxide (CO), even well below permissible national limits, may hasten hospitalisation of elderly cardiac patients, according to a new study in the US.

The nationwide study of 126 urban communities found that an increase in CO of one part per million (PPM) in the maximum daily hour-long exposure is linked with a 0.96 percent hike in hospitalisation risk from cardiovascular disease among people over 65 year.

This link holds true even when CO levels are less than one PPM, which is well below the National Ambient Air Quality Standard of 35 PPM in the US.

This finding suggests an under-recognised health risk to seniors. Currently, the Environmental Protection Agency (EPA) is evaluating the scientific evidence on the link between CO and health to determine whether the health-based standard should be modified.

'This evidence indicates that exposure to current carbon monoxide levels may still pose a public health threat,' said Michelle Bell, the study's lead investigator and associate professor of environmental health at the Yale School of Forestry & Environmental Studies.