Matt Haworth was paying a visit to a sex worker charity in Manchester when a brightly coloured bulletin board in the corner caught his eye. It was covered with descriptions of bad punters – those who were abusive with sex workers, or didn’t pay up. “One that really stuck with me was a man who drove around in a Vauxhall, throwing hardboiled eggs at sex workers,” Haworth tells me over the phone, several years after the event. “It preyed on my mind for years. Why did he hardboil them?”

There are around 80,000 sex workers in the UK, and they’re statistically more likely to be attacked or raped at work than most other groups. Because of their unsure footing in a country where sex work isn’t criminalised, but many related activities like streetwalking or running a brothel are, sex workers are also unlikely to trust the police – and police can be reluctant to help, or keen to clamp down on the profession rather than protect its workers.

The board Haworth saw in Manchester was an analogue version of National Ugly Mugs (NUM), a service run by the UK Network of Sex Work Projects. Now, it protects sex workers from rogue customers via a network of text and email alerts that are tailored to specific regions. The service gave Haworth, who owns a technology company, an idea: what if the sex workers could get these alerts directly to an app, and also use it the app to report back on their own safety?

With his team, Haworth developed the NUM app based on the charity's body of knowledge and feedback from sex workers themeslves. Spreading alerts as quickly as possible is a vital part of the app's offering. As Haworth tells me, the need for it is aptly demonstrated by the case of Thomas Hall, who attacked four sex workers in the course of a single evening in Manchester in 2013. This feature was also inspired by location-based dating apps like Tinder and Grindr. “We wanted to use the same location technology for a very different end,” Haworth tells me.

The app checks incoming numbers with its database of rogue punters, and also features a kind of panic button, which workers can press if they feel unsafe. Again, detail is key: the button feature uses a black background, so the phone doesn’t light up sex workers' faces and attract attention. The button can be used to report bad clients, call the police, or log that the worker felt unsafe so NUM can check in with them later to offer services and support. The app has been tested in Manchester to a positive response, and is currently undergoing a bigger pilot in London. Haworth tells me that the police themselves are supportive of the scheme.

This would all be moot, of course, if smartphones weren't already part of sex workers' lives – but Haworth found out in focus groups that “many said that the internet and technology were paramount in their work”. Reason Digital, Haworth's company, carried out what he believes is the first dedicated research into sex workers’ smartphone use, and found that somewhere between 30 and 40 per cent of sex workers in Manchester use a smartphone. Anecdotally, Haworth found that escorts and “indoor workers” who don’t walk the streets are more likely to use them, partly because “they get bored – there’s lots of waiting around”.

In fact, over the past few years, there has been a rise in technology services marketed specifically to sex workers. German site Peppr was billed earlier this year as the “Tinder for sex work”: workers can advertise their services, and punters can contact them through the app.

Unlike Ugly Mugs, it’s purely for advertising, and isn’t particularly concerned with workers' safety. I asked a customer service representative if the app acts on reports of violence, and was told that the company reserves the right to block any user, but has only done once for a no-show. “It’s amazing how effective linking people to their address and payment card is,” the representative told me.

Image: Peppr

The rise of apps aimed at sex workers isn’t surprising when you consider that sex workers have used online advertising for about as long as the internet has existed. Margaret Corvid, a New Statesman blogger who works as a dominatrix in Plymouth, tells me that she does all her advertising online on sites like Adult Work (she also receives NUM email alerts and reads them “religiously”).

In the early days of the internet, sex workers used directories like Alta Vista to list their services. Some of these have even survived the rise of Google and are still used by some workers, Corvid tells me, “especially in kink”. Many sex workers advertise, or have advertised, on sites like Craigslist or even Facebook, but these companies have become stricter in shutting down sex work advertising.

Craigslist originally ran an "Adult" listings section, but closed it in 2010 under pressure from the public, yet Corvid argues that the ability to advertise and receive payments online actually makes sex work much safer. Clients email her, then she “insists on a phone call with every client” and takes a security deposit via online payment.

In the US, where sex work is still criminalised, major credit card companies are pulling their services from sex work sites, and in doing so, putting sex workers at risk. This is partly because the ability to advertise online means workers can act alone. “You don’t need a manager or a pimp, and you can set your own prices and choose your own clients,” Corvid says.

Apps like Peppr, which automate the transaction, could arguably make this process less safe, however. Their click-and-go business model doesn’t encourage the kind of screening processes Corvid uses, and the app doesn’t pre-screen clients either.

Online booking and advertising also results in a digital paper trail, which, depending on your jurisdiction, can be a good or a bad thing. In the US, where the law is harsher on sex work, a digital footprint can also be a risk for workers and punters alike. In the UK, it may actually make the work safer. “Right now it's a good thing there's a paper trail, because even though it's almost impossible to get the cops to deal with issues of assault and violence against sex workers, there would be at least some records of the punter through the app system which could be obtainable by authorities,” Corvid says.

Apps and websites, whether they are for safety or advertising, also offer other, less obvious, benefits for sex workers. “Sex work is a historically isolating occupation,” Corvid tells me, “and technology has really changed that.” Technology allows workers to organise politically when needed, or just swap tips – “like ‘Where do I get this specific type of stocking my client asked for?’”

This was one aspect of sex workers' use of technology that surprised Haworth and his team while they were developing the NUM app. At one meeting, Haworth tells me, a male sex worker in his teens asked quietly: “Are you only going to send out bad news? What about good news?” As a result, the team are including news of new support groups and successful convictions of rogue punters in their updates.

Overall, both old-school listings sites and apps aimed specifically at sex workers are empowering a group traditionally maligned by society, the police, and even, on occasion, its own clients. As Haworth tells me, the NUM app is radical because it’s “decentralised – it lets sex workers look out for each other”. Until our more traditional instiutions get their act together in their dealings with sex workers, this will remain incredibly important.