We've all seen them: work colleagues who refuse to stay home when they're sick, no matter how contagious. Most of them think they're doing their workplace and themselves a favour, but a new study from Concordia University finds workers who go to work ill often aren't helping anyone.

Gary Johns, a management professor at Concordia's John Molson School of Business, has just published a new study in the Journal of Occupational Health Psychology on "presenteeism," or attending work when ill.

He says while absenteeism gets a lot of attention, presenteeism hasn't been as well studied.

"I was interested in why would somebody go to work when they're ill and what were the predictors of that," Johns said in a phone interview.

So he surveyed 444 recent business school graduates an average age of 31 about presenteeism. He asked them about their jobs and work experience and how often they called in sick to work, and how often they went into work even when they knew they weren't well.

Over the previous six months, the volunteers reported an average of three presenteeism days and 1.8 absenteeism days.

And what did Johns learn about employees who show up for work even with illnesses such as colds, migraines or back pain? A number of things guide their decision, including how important they felt their work was to the organization.

"People were most likely to show up for work when they were engaged in teamwork or project work or anything where they might let their colleagues down by not coming in," Johns explained.

"They were also inclined to go to work whenever they saw their work as having a significant impact on clients or customers or maybe patients," he said.

Perhaps not surprisingly in these economic times, those who were worried about their job security were also more likely to go into work even when sick, the study found, as were people who worried that they could be easily replaced.

"Also, people who had a positive view or were most liberal about absenteeism, they obviously weren't too inclined to engage in presenteeism. And when they did, they weren't too productive," Johns found.

Johns notes that while it might be assumed that presenteeism results in unproductive workers who essentially waste their work day, that's not always the case.

He found that people who scored high on a standard personality test for conscientiousness tended to report that they were still productive even when they worked while sick.

But Johns notes that other research has shown that presenteeism can lead to "downstream" absenteeism.

"In other words, you could start with a small health problem that you exacerbate by going to work and then you have to stay home and you don't have any productivity at all," he says.

Johns says workplaces and human resources departments have traditionally examined ways to curb absenteeism, but have paid little attention to presenteeism.

He says it's important to do so by encouraging employees to take sick days when they're not feeling ill, both for their own sake and for the sake of the workplace as a whole.