(CNN) -- As freight railroad workers look to Congress to provide them with paid sick days, millions of other American employees have no safety net if they fall ill.

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The US does not have a national standard on paid sick leave, a rarity among industrialized nations. Roughly 1 in 5 civilian workers lack paid sick days, but the prevalence of the benefit varies widely by occupation and wage, according to federal data. Public sector workers, management and professional employees and higher-earning staffers are more likely to have access to paid sick days.

However, many others are not as fortunate. Roughly one-third of workers in service, construction, extraction and farming occupations don't have paid sick days. About half of part-time workers lack the benefit, as do more than 40% of those in the lowest quarter of wage earners.

Overall, about 33 million workers have no paid sick days, according to Family Values @ Work Action, an advocacy organization.

The lack of paid sick leave hurts both workers and the economy, said Seth Harris, professor at Northeastern University and former top labor policy adviser to President Joe Biden.

Americans with disabilities or chronic conditions may be less likely to accept positions if they don't come with paid sick time, which creates structural barriers to employment, he said. And companies aren't required to offer the benefit, forcing some people to come to work sick or face losing pay or their jobs.

"The most disempowered workers -- who are low-wage workers -- don't have an opportunity to demand paid sick leave from their employers," he said.

Also, people who come to work when they aren't well are more likely to spread their illness to others. This became a major problem during the early part of the Covid-19 pandemic among essential workers who lacked paid sick days.

"It's a bigger issue of trying to take care of the entire workplace by making sure people know to stay home and that they have the flexibility to stay home if they are sick," said Emily Dickens, chief of staff at the Society for Human Resource Management, a trade association.

Railroad workers' battle

While the vast majority of union members have paid sick days, the freight railroad workers do not. Among other demands, they have been threatening to strike in order to get paid sick days that are not in the current contracts.

The railroads say that workers can use personal time if they need a sick day. But the unions argue that with current staffing levels and scheduling rules, it's difficult for workers to have personal days approved, and they are likely to be penalized or even fired if they call in sick anyway.

What benefits the rail unions obtain is now up to Congress, after the tentative deals -- which lacked paid sick leave, but included lucrative pay increases -- were rejected by the rank-and-file members of several unions. That led Biden on Monday to call for lawmakers to pass legislation averting a rail shutdown by officially adopting the tentative agreement approved by labor and management leaders in September.

Progressive lawmakers, however, demanded to add paid sick days to the contract.

"At a time of record profits in the rail industry, it's unacceptable that rail workers have ZERO guaranteed paid sick days," Vermont Sen. Bernie Sanders, an independent who caucuses with Democrats, tweeted Tuesday. "It's my intention to block consideration of the rail legislation until a roll call vote occurs on guaranteeing 7 paid sick days to rail workers in America."

The House passed the tentative rail agreement on Wednesday, with 79 Republicans and most Democrats supporting it. Lawmakers also voted separately to add a provision that would give the workers seven paid sick days, which was backed by nearly all Democrats but only three Republicans.

It's unlikely that the provision will garner the votes needed in the Senate, which can pass the original rail agreement without the sick leave measure. At least 10 GOP senators would have to support providing paid sick days to the rail workers.

Government mandates

The Covid-19 pandemic focused renewed attention on the lack of paid sick days, as well as family and medical leave, in the US.

In one of its early relief bills in 2020, Congress created temporary paid sick and family leave benefits for many workers who contracted or were exposed to Covid-19, were caring for family members affected by the virus or had to care for their children after schools shut down.

However, the pandemic failed to convince Congress to pass legislation making the benefits more widely available to workers on a permanent basis.

Biden had called for a federally funded paid family and sick leave program for those who don't receive it at work as part of his hefty Build Back Better package in 2021. The House passed a bill last year that included four weeks of paid family and sick leave, which would have cost $205.5 billion, according to the Congressional Budget Office. But the provision didn't make it through the Senate.

Meanwhile, a growing number of states, cities and counties have been enacting paid sick leave laws in recent years.

Some 14 states and Washington, DC, have specific laws on paid sick time, while Maine and Nevada have more general paid time off laws that allow covered workers to use the benefit when they are ill, according to A Better Balance, an advocacy group. Also, 20 cities and counties have approved paid sick time laws.

Nearly 57 million workers are now covered by state or local paid sick time laws, according to an analysis by A Better Balance.

Advocates see the railroad workers' battle as an opportunity to renew interest in expanding the availability of paid sick leave.

"Workers should be able to take for granted that they can stay home and take care of themselves if they're sick," said Mia Dell, deputy director of advocacy at the AFL-CIO, a federation of labor unions.

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