The southern California sky dims as Vicky Márquez zooms south along Interstate 5 in her Honda SUV, with syrupy Spanish-language love songs blasting from her stereo. The satnav on her phone is directing her through a monotonous landscape of Orange County office parks, and Márquez is racing against rush hour, dodging between lanes and swerving with inches to spare. “I’m kind of a crazy driver,” she admits.

Márquez works for a little-known non-profit organisation with the pressing goal of fighting labour exploitation among night-shift janitors – an industry that operates in obscurity, with workers sent to anonymous buildings rarely visited by government regulators. With her glasses, curled-under fringe and pastel sweater, Márquez looks more like a retired librarian than a labour rights activist. On tiptoe, she stands under 5ft tall. On this particular late winter evening, Márquez is on the road to the first of half a dozen office parks where she will make surprise visits, making sure that cleaners are being treated fairly by their bosses.

It is a job that few government agencies bother to do, but it is work that Márquez believes in. For 16 years – after moving to the US from El Salvador, where she left her husband and three children behind – she too worked as a janitor. The work was rough, and she had to put in more hours than she was paid for, but she still managed to send money back home.

After 40 frenetic minutes on the road, Márquez arrives at her first destination, near the city of San Clemente. She climbs out of the car carrying a bulging black bag stuffed with papers and tests the front door of the office. Tonight, she has arrived early enough that the door swings open. Márquez has other strategies for when they are locked: she might station herself near the service exits or the dumpsters, where she knows the night-shift cleaners will eventually present themselves. In supermarkets or guarded high-rises, she will sweetly ask for the janitor. If the person she is talking to assumes that she’s looking for a job, so be it.

Tonight her first move is to look for bathrooms or supply closets – two places she knows she is likely to find a janitor. She moves past the elevators to a rear hallway, where she finds María García, a cleaner, holding a mop next to a bucket of murky, citrus-smelling water. Márquez greets her in Spanish. García is on the clock and responds brusquely, almost warily. Márquez doesn’t waste time on small talk. Setting her large bag on top of a drinking fountain, she extracts a packet of papers and passes them to the cleaner. Márquez explains that she works for an organisation called the Maintenance Cooperation Trust Fund (MCTF), which helps janitors make sure they get paid what they are owed, and helps them solve problems with issues such as immigration.

When she has García’s attention, Márquez asks a few more questions: does García get paid in cash, or with a cheque on a regular basis? A cheque every two weeks, García says. Márquez nods. Is she given regular breaks? Yes. Does she have to pay for her own cleaning supplies? García says that sometimes what the company gives her is not enough, so she has to buy a few more bottles of bleach. Márquez tells her it is the company’s responsibility to provide her with the supplies she needs.

Then Márquez goes in to close the deal. “Tu teléfono, mija?” Márquez asks. Márquez scribbles the number into a black notebook. “Y tu dirección?” Márquez then takes down García’s address.

Gathering workers’ contact information is Márquez’s ultimate aim. The MCTF is one of only a handful of organisations in the US keeping careful tabs on the practices of non-union cleaning firms – some of which operate entirely in the black market. Through these impromptu meetings, the organisation has generated a database of workers who can give first-hand testimony about whether these companies are following labour laws. Since 1999, the MCTF has helped collect more than $26m (£19m) for janitors who were being abused at work.

García doesn’t know it yet, but Márquez will later call or visit her at home in the early afternoon, when most night janitors have not yet left for work. At these follow-up meetings, Márquez will remind García that she is there to help her solve any problems she may be facing at work. If García doesn’t pick up or answer the door, Márquez will keep trying until she makes contact with her a second and then a third time. This process can take months, but such is the long, slow dance necessary to build trust among workers in low-paying and invisible industries – people who are unlikely to ever make formal complaints.

A 2016 protest in Sacramento, California, in support of a bill to protect female workers from sexual harassment. Photograph: Rich Pedroncelli/AP

As a reporter who has investigated these industries for several years, I have been forced to conclude that low-wage immigrants labouring in isolation are at unique risk of sexual assault and harassment. It is an open secret in these industries that immigrant women in financially precarious jobs – many of whom are undocumented – are targeted for sexual abuse by their superiors. While it is not possible to know how often these abuses happen, they are not anomalies. Federal government figures estimate that about 50 workers are sexually assaulted each day, and in the industries that hire newcomers to the country in exchange for meagre paycheques, such assault is a familiar workplace hazard. And yet there have been few meaningful efforts to prevent it before it starts. Instead, we expect women with the most to lose to seek recourse by reporting the problem after the fact – but the reality is that if these workers face abuse from a superior, the combination of uncertain immigration status, financial constraint and shame often conspires to keep them silent.

The same unfortunate pattern plays out among farm workers and domestic workers – those who cook, clean and care for families behind the locked doors of private homes. Their vulnerability to sexual violence echoed what I had heard from janitors: in their isolated workplaces, it was often their direct employers who groped them or propositioned them for sex.

In her work as an advocate for cleaners, Vicky Márquez has discovered that a lot can happen in places no one is looking – but she didn’t realise the full extent of it until she met a young cleaner named Georgina Hernández.

Many low-paid jobs share similar risk factors. Every day vast numbers of women – often hired via a complex web of barely accountable subcontractors – find themselves working in isolated locations across the US. With few other people around – and those that are often battling poverty and eager to keep their jobs – they can become the perfect target for predators.

Hernández was working at a cinema sweeping up popcorn when Márquez first met her. For the first month and a half on the job, Hernández never received a pay cheque, and worked more hours than she was being paid for. She hadn’t complained because she thought that, as someone without immigration papers, she was easy to replace. She still recalls the way Márquez spoke to her – gently, like an understanding aunt. Before Márquez left, she took down Hernández’s phone number and told her she would check in with her again.

On her next visit, Márquez looked at Hernández’s pay stubs and compared them to the working hours she had written down in a notebook. They didn’t match up. When Márquez asked why Hernández hadn’t written down any time for breaks, she said it was because she wasn’t given any. Márquez and the MCTF eventually helped Hernández and some of her co-workers file a complaint with California’s labour commissioner, which led to a $1m fine from the state of California against the two cleaning companies that had employed Hernández: for failing to pay minimum wage and overtime, and for not giving their workers rest or meal breaks.

But when Hernández moved to a new, higher-paying job as a hotel cleaner, there were even bigger problems. Early on, she says, her supervisor flirted with her and tried to convince her to have sex with him. She rebuffed him, and he retaliated by giving her more work. When his advances didn’t stop, she tried to hide from him, but he would follow her or call her on her cellphone to find out where she was.

His demands quickly became violent. Less than a week into the new job, Hernández says he told her that he needed to talk to her privately about her work in his car. This made her uncomfortable, but he said: “You need this job, don’t you?” He instructed Hernández to meet him in the parking garage. Worried about losing her job, she went. When she got there, he told her to get into the vehicle. She hesitated, but he was the boss. She did what she was told. The supervisor drove them to a higher, darker floor of the garage. After he parked, he began to touch her legs. She told him she didn’t want to continue, and he replied that he would give her more days off and better pay. Hernández told him she didn’t want more days off – she had taken the job because she wanted to work. When he began touching her breasts, she became afraid. Then he took off her trousers. As he forced himself on her, she panicked and froze.

Afterwards, the supervisor asked her to put in a request for an extra shift that week, so he could take her to a hotel. Hernández told him she couldn’t. He assured her there would be perks: he would pay her for the shift, and make sure she received seven shifts that week. “You’re delicious,” he told her before driving her to a lower level of the parking garage. He told her to go into the building first. He followed a while later.

Hernández never requested an extra shift. She didn’t immediately tell anyone what her supervisor had done. The shame of it was too much, and she knew it would not be easy to find a new job as an undocumented worker who couldn’t read or write.

About a week later, Hernández’s supervisor told her to meet him again. When she said no and tried to quit, he threatened to hurt her and her daughter. He added that if she wanted to stay in the country, she needed to keep him happy. This time he drove them to a motel.

On one of her nights off, Hernández’s supervisor called her incessantly until she picked up the phone. He said he needed her to work that night and that he was on his way to pick her up. Hernández hurried to get herself ready for work, but once she was in his car, he drove to the motel. Hernández cried and tried to climb out of the car, but he pulled her into a room by her hair, where she says he forced her to have sex with him again. He later warned her not to tell anyone what had happened – but she would have stayed silent anyway. She thought her family and friends would never believe her, or would think she had brought it on herself.

Hernández says that, at the time, she didn’t think there was a way out of her supervisor’s trap. “There’s no way to defend yourself,” she says. “There’s no way to say no. When you need the job, you become the victim. That’s why you deal with all the harassment, the discrimination, everything – because you need the job.”

For the next few weeks, she reported to work at the hotel as usual, making a point to avoid her supervisor. But he managed to find her, either to remind her how much she needed the job, or to chastise her for being so cold during their encounters. Finally, he came to her with an ultimatum: she had to decide whether she wanted to keep her job or not. If she did, he would continue to have certain expectations of her.

Hernández felt hopeless. She was having migraines and panic attacks. She dreaded his next demand. When he confronted her again, she told him she would not have sex with him to keep her job. Then began his revenge. He yelled at her in front of co-workers and disciplined her for supposedly leaving used tissues in the lobby. Then, she says, he started to sabotage her work, making a mess of places she had cleaned and disciplining her for it.

The rapes had been horrific violations, but they had happened in private. Now her supervisor was publicly impugning her work, and her job was still at risk. She felt lost and compromised, but she swallowed how she felt and continued to drag herself to work.

She later decided to speak to the cleaning company about the assaults. She had seen her supervisor try to hug and flirt with another cleaner, so together the two women called human resources to make a complaint, but nothing changed. Almost two months into the job, Hernández called in sick one evening. The next day, the supervisor fired her.

Hernández didn’t leave the house for days. She had headaches and couldn’t sleep. Her nausea continued to intensify. The truth was hard to face: she was pregnant. Depleted and sobbing, she sought out the only person she could trust. Vicky Márquez remembers how Hernández sounded that day. She was crying and her voice was anguished. Márquez had rushed to Hernández’s apartment, but the cleaner said they could not talk there. “I don’t want anyone to hear me,” she said.

They talked in Márquez’s car. “Something has happened that I don’t want to have to tell you, Vicky,” Hernández began. “Something terrible.” She was inconsolable. Márquez told her: “We can find a lot of help for this. Don’t be scared.” But in truth, Márquez had no idea what she could do. She called Anel Flores, a colleague who was an attorney, to ask for help. Flores suggested Márquez bring Hernández to their office. For the next two hours, Hernández shared fragments of her experience, until Flores was able to piece the whole story together, from the rape in the parking lot to being forced into sex at the motel. Finally, Hernández told Flores that she was pregnant from one of the rapes, and that she had already made an appointment for an abortion. “I cannot have this baby,” she said.

She added that she was worried people would find out she was pregnant, and that she would be judged and blamed for everything. Flores tried to reassure her: “It’s not your fault. You did not do anything wrong. You’re not a bad person.” Hernández eventually agreed to let Flores share some of what had happened with Márquez, and with Lilia García, the executive director of the MCTF. They told Hernández they would help her address the problem step by step, and would become her confidants and support network.

When Hernández terminated the pregnancy a few days later, Flores picked her up from the clinic and took her home. After an attorney specialising in sexual harassment suggested that Hernández file a police report, Flores and Márquez drove Hernández to the police station, and Flores sat with her as she was interviewed.

Reassured by the support of the women from the MCTF, Hernández became determined to push back against what had happened. With the help of the lawyer she had met through the organisation, Hernández filed a sexual harassment lawsuit against the cleaning company. In the legal filing, Hernández accused the company of failure to prevent sexual harassment, wrongful termination and retaliation, negligent supervision, intentional infliction of emotional distress, and assault and battery. Within months, the company paid a financial settlement to close the case, though it did not admit any liability in the process. It also fired the supervisor.

Hernández says the outcome of the case can never make up for the rapes, but she is proud that she set aside her fears to challenge what had happened to her. She had tolerated too much for too long because she didn’t know how to get help, and she might have been stuck with the same problems if Márquez hadn’t found her. “I would have guarded all of this pain,” she says. “I wouldn’t have known how to speak out about what happened.”

Márquez says the isolation of the job and the demographic of the workers makes night-shift cleaners such as Hernández easy targets for abuse. “It is because the supervisors always think the worker needs work and they have work to give,” Márquez says. “So they commit these abuses. And there are many – who knows how many hundreds or thousands of cases – that remain in the shadows because no one knows. Many women don’t say anything out of fear. They’re afraid that society will realise that they have been forced to sleep with someone. They are afraid that they will lose their job.”

Márquez knows that it is rare to uncover cases such as Hernández’s. For each janitor the MCTF reaches, it can take months of calls and visits before a worker will begin to think about speaking up about their problems. In matters of sexual assault, it takes even more work and time. “How many cases are there in this country that we don’t know about?” Márquez says.

Across the US, immigration status and poverty are used as leverage against female workers to hold them hostage in jobs where they are being sexually abused. Labour enforcement is predicated on the idea that workers already know their rights, and thus it is logical to expect them to make a complaint to bosses or the government if problems arise. These laws do not take into consideration the experiences of low-wage immigrant workers and what their options really are if they have been sexually assaulted at work.

The legal system – through filing a civil lawsuit or a criminal case – is often viewed as the clearest way to demand accountability. Workers can also go to their employers or unions to demand redress. Making a formal complaint helps emphasise that there can be consequences for this type of abusive conduct. But these approaches are only part of the solution, and are inherently reactive, requiring the confrontation of systemic roadblocks – such as deeply flawed notions of credibility – that often get in the way of satisfactory outcomes. Meanwhile, we know that prevention is possible. Decades of empirical research offers clear direction. While there are some heartening efforts to incorporate this research into worker training and advocacy programs, employers and policy makers have largely chosen not to use it.

Supporters of the Justice for Janitors campaign at the Sacramento protest. Photograph: Rich Pedroncelli/AP

In addition, advocates for female workers have tried for decades to make the case that sexual assault at work should not be dismissed or marginalised by employers and the government because it has historically been perceived as a “women’s issue”. Instead, they argue, gender-based violence should be viewed in the same way as other forms of on-the-job physical violence, so that prevention plans are implemented, the government takes a proactive role in enforcement and workers have an avenue for demanding accountability.

Sexual assault can happen to anyone, anywhere, but if there is a perfect storm of factors that put workers at risk, night-shift janitorial work is its epicentre. Nearly every office building relies on after-hours cleaners, but we rarely see the people who do the vacuuming and mopping. The work is scheduled to happen at night or during the early morning, when few people are around. They are expected to be invisible.

This is also emblematic of a wider trend. Before the 1980s, most businesses had their own cleaning staff. Then, as institutional investors purchased high-rises and retailers grew into chains with premises all over the country, it became more efficient and cost-effective to outsource such work to independent companies. A wide range of low-paid, unskilled industries have followed suit.

This business model has become even more opaque with the rise in subcontracting. While one contractor might land the official cleaning contract for a large retail chain or high-rise office block, it might then hire a subcontractor to do the actual cleaning. Some of those subcontractors might then subcontract some or all of the work to a third business. Building owners, retailers and businesses award contracts to the lowest bidder, so cleaning companies – both big corporations and small subcontractors – have to keep costs as low as possible. Human labour is the largest expense in this business, and where the firms look first to trim costs.

“The way you make money in this industry is to cheat, because the profit margin is so thin,” says Stephen Lerner, who led the Justice for Janitors campaign with the Service Employees International Union in the 1980s. Larger companies are not without their problems, but they are easier to track, and most provide regular pay cheques and benefits to workers. But at the other end of the spectrum are an unknown number of black-market subcontractors, where misconduct largely stays hidden.

In many ways, the MCTF is doing for cleaners what the state could be doing for all vulnerable workers. “The reality is that there are very few or no enforcement agencies who do this work,” says Lilia García, underlining the fact that the many of the cases her organisation brings would probably go unreported if the job were left solely to the government.

In some states, including California, there is a push to create regulations regarding workplace violence that would address everything from physical attacks to sexual assault. At the federal level, the US Occupational Safety and Health Administration (OSHA) acknowledges that it has a responsibility to address sexual assault on the job. In reality, though, the OSHA doesn’t do much to tackle the issue. It took on its first case of workplace sexual assault in 2016.

For now, it is up to organisations such as the MCTF to do the tough work of finding the cases that no one else is looking for. “There is not [much information] in these work sites of where to call if you’re in trouble, of what you can do if a right is violated or if you’re attacked,” says García. “They were almost like these lost islands, just operating in the middle of the night, for years and years and years. We’re actually connecting them with society, and letting them know that their working conditions are wrong or that an attack on their person was wrong and there’s something that they can do about it.”

Sexual assault in the workplace is a crime and an extreme form of sexual harassment. It is outlawed by the US Civil Rights Act of 1964. Nevertheless, companies do not have to disclose how many sexual harassment complaints they receive internally, whether the claims were physical and violent, or how they handled them.

Complaints made to government agencies are often kept confidential until one party decides to file a lawsuit. Most sexual harassment complaints received by the federal government never result in litigation. They are processed and then stored in filing cabinets or databases.

Even lawsuits don’t always reveal much about what is really happening. If a worker threatens to file a sexual harassment suit, the company can buy the person’s silence by offering a confidential settlement before the case is filed and becomes public information. Cases that do make it to court can be kept under wraps through quick settlement agreements, which include confidentiality clauses that silence the worker and sometimes their attorneys.

Of course, some workers don’t want their personal business known to everyone. Meanwhile, companies argue that keeping these claims out of the public eye is necessary. They say they settle cases as a way to end an embarrassing complaint, even when they don’t truly believe the harassment happened. As a result, they worry that these lawsuits can sometimes become a kind of extortion by disgruntled or dishonest employees.

Advocates such as García, however, argue that it is difficult enough to convince women to come forward about far less sensitive problems – being paid less than the minimum wage, for example. On the dozens of occasions when her organisation has unearthed cases involving sexual violence, the abused workers, for the most part, haven’t wanted to move forward with formal complaints because they didn’t want anyone to know what had happened to them.

“They internalise the shame and the wrongdoing, and the embarrassment is just overpowering,” García says. “They choose not to talk about it to any of their relatives. They really have no other support outside of whatever our organisation can provide.”

When sexual assault happens among invisible workers in industries that few are monitoring, it becomes a crime that can be denied, a problem that never receives accountability or prevention. The repercussions of ignoring the realities of vulnerable workers are clear: if on-the-job sexual violence rarely comes to light, then the problem goes unaddressed and the perpetrator is free to abuse again.

This is an adapted extract from In a Day’s Work: The Fight to End Sexual Violence Against America’s Most Vulnerable Workers by Bernice Yeung, published by The New Press. Buy it at guardianbookshop.com

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