Donating blood is ok and so is storing in anticipation. But storing donated blood too long increases the chance of an infection.
The risk of blood poisoning or pneumonia doubled once the 29-day mark passed, Cooper University Hospital in New Jersey, US found.
The study, presented at a US conference, calls for increased care over the way blood is used and stored.
UK authorities said blood took 10 days on average to reach hospitals and daily deliveries meant it was not stockpiled.
The US does not allow the use of blood stored for longer than 42 days - in the UK this is lower, at 35 days.
After two weeks in storage, red blood cells start to undergo changes which lead to the release of chemicals called "cytokines".
These are known to hinder immune function, and in high levels could possibly make patients more susceptible to infection.
Researchers looked at the rate of hospital infections in 422 patients against the age of the blood transfusion they received.
They found that the average age of the blood was 26 days, and 70 percent of patients had received blood older than 21 days.
In total, 57 patients developed an infection - and these patients had received older blood than the others - on average it had been stored for three and a half days longer.
Patients who had received blood older than 28 days were twice as likely to develop an infection, and the more units of blood given, the higher was the chance of infection.
Dr David Gerber, who led the research, and presented the results at the American College of Chest Physicians conference in Philadelphia, said that any change to the time limit could lead to a blood shortage.
"More cautious utilisation of blood might help to alleviate, at in least part, a diminished blood supply that might result from such a change in policy." about
A spokesman for the National Blood Service said that UK hospitals were not as reliant on using older blood stocks.
"The shelf life of blood in the UK is shorter than in the US."
She said that the average age of blood arriving at hospitals was just over 10 days, and daily deliveries meant that hospitals were less inclined to stockpile blood.
She said: "We continue to work hard with hospitals to improve blood stocks management and ensure a safe, sufficient supply of blood to meet patients' needs."