Mexico City, Mexico - Unseasonal rain is tipping down outside the diner by Mexico City's Benito Juarez airport. The brake lights reflected in the puddles on the wet tarmac make them look like pools of blood. The air is one huge engine roar: planes above, cars all around.

With its bottomless coffee, red banquette seating and dim yellow lighting, the diner could be the scene of a 1950s noir movie. It's a fitting choice for the suited man with the silver crew-cut sitting alone at the foot of the room.

Forty-three-year-old Donovan Tavera is Mexico's only government-certified forensic cleaner. Since launching his private company 16 years ago, his work has taken him all over Mexico into the darkest, loneliest corners of a country locked in a militarised war on drugs and swept away by rapid, voracious modernisation.

"Because I'm the first, I have to be the best," he says, lighting the first of an endless chain of Montana cigarettes. He uses matches because lighters accrue too much dirt. "That means I have to make the most personal sacrifices."

Still, he says, "The credit for the job goes to every doctor or pathologist who answered my questions, or who lent me their laboratory. I didn't invent this: I just brought together the information."

At his home laboratory in Texcoco, in the state of Mexico, Donovan has invented some 370 chemical formulas to eradicate traces of murders and suicides, home fires and abandoned bodies.

"My goal is for every place I work in to appear as though nothing tragic ever happened there," he says, his gaze fixed on the coffee pooled in his cup like the pupil of an interrogating eye.

"My formulas make the difference between me and other people who try to do this," he explains. "So many office and domestic cleaners get contracted for jobs that are beyond their capacities: They simply throw bleach and water on the material and their clients believe the work is done. This is deceit, as far as I'm concerned. Blood is dangerous.

"Just because the stain and odour have gone, you may still have tuberculosis, HIV, hepatitis live at the scene without a deeper chemical intervention. Stabbings sometimes leave blood mixed with pericardial fluid that requires one particular formula. The spatter patterns spread after a shooting require various different preparations, since a bullet cuts through various different tissues. Where you have an abandoned body, you may have acid and other digestive fluids, so, once again, I have to make a different formula."

Donovan speaks in complete paragraphs, in a tone of quiet authority, but the voice emanates from a heavy-set physique that is a testament to his previous careers as a soldier and a private bodyguard.

The money would be better in either of those professions, he says, but his motive is personal, not commercial.

"In the United States, I could charge $400 per hour," he explains, cleaning his fingers on a napkin. "This figure is the maximum I can ask for here [for the entire job]. When you take into account my expenses - the chemicals, transport, my assistant's wage - I have a very slim profit margin."

'Where does the blood go?'

His career is the culmination of a lifelong obsession dating back to his childhood in 1980s Azcapotzalco, a gritty industrial area of Mexico City, smothered under a blanket of smoke from the Pemex refinery that dominates the neighbourhood.

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"Since I was 12, I've asked myself the same question," he says. "Where does the blood go?"

"One Saturday afternoon, I looked down from our apartment to see the victim of a hit-and-run lying dead beside a taco stand - just 200 metres away. His blood was pouring into the gutter. I asked my mother where this blood would go, but she had no answer. The following morning, I watched the taco stand owners rinsing away the blood with soap and water. By midday, it was business as usual, queues of people standing eating tacos where the body had lain. No ritual, no nothing. The event had been rendered invisible. I felt so sad.

"When we talk about this as a family, my mother talks about how I was traumatised by what I saw that day. Maybe I was, but I know that this gave me the impulse to start my life's work. I decided that if nobody else was going to care about these things, I would. That's why I spent my adolescence in the library and in our garage, trying to find out how to replicate the precise texture of blood - not the film effect stuff, the real thing - at the level of chemical composition and so on. When I mixed my first formula, and it dissolved the fake blood I had made, I knew that I had a future as a forensic cleaner."

"Something like Sherlock Holmes?" I ask him.

"Maybe a little," he chuckles. "I love those books, even though they're not very realistic. A Study in Scarlet is my favourite one, because you get to see how he learned everything he knew. And, of course, this fantasy that there is a solution possible for every problem always appeals to me."

'I arrive, I work, and I disappear'

Being self-taught is in part a personal preference for Donovan . "I've always been pretty independent. I left the army when I was 23 because I just didn't feel stimulated intellectually. Working as a bodyguard gave me more freedom in that regard," he says. But it is also because, as he puts it, "forensic cleaning doesn't exist in Mexico".

"I learned almost everything myself. I was nearly 30 years old before I spoke to a pathologist, and that was my wife's idea. Accessing that type of profession can be complicated. There is still quite a frozen social hierarchy here," he says.

Although his work has been covered in the English, Spanish and national media, there are moments in our conversation when Donovan cuts an isolated figure.

"We don't have a culture of due diligence in Mexico," he says, looking physically deflated. "Most of my work is cleaning up after the police investigating the crime: fingerprint dust, boot-prints, even scraps of evidence that they've rejected or ignored."

While forensic sciences are enjoying a roaring trade in Mexico, with hundreds of students subscribing annually to courses at prestigious universities and private institutions, the application of these sciences is behind the pace of the violence sweeping the country. The country's forensic services are simply overwhelmed by the sheer number of bodies.

According to the executive secretary of the National Public Security System (SESNSP), 57 people have died from violence each day in Mexico this year. This figure represents an increase of 15 percent on the toll for the same period last year [PDF].

According to another government institution, the National Institute of Statistics and Geography (INEGI), only one percent of all crimes reported to the authorities are resolved.

READ MORE: Flawed forensics

Forensics bear the brunt of this systemic failing. According to an AFP report published in July, mortuary freezers in Acapulco, Guerrero State - the country's most violent city - are literally overflowing with the bodies of murder victims. One container with a capacity for 95 bodies was found to be holding 174 cadavers: almost double its limit.

The situation seems to invite comparisons between the Mexico of 2016 and the country as it was during the violent post-revolutionary decades, when its first forensic services were founded in order to alleviate the pressure on public hospitals.

Donovan 's lonely, almost invisible work is a reflection of a country struggling to establish the rule of law.

Edgar Elias Azar, the president of the Mexico City Court Service, whose organisation oversees forensic science instruction in the nation's capital, says that Donovan "is doing important work in a field that leaves a lot to be desired".

"If people begin to recognise the value of what he is doing to prevent health risks, forensic cleaning will be taken a lot more seriously. But for the moment, he is all we have," he continues.

Indeed, forensic cleaning is so unrecognised in Mexico that Donovan had to write his own paperwork when seeking to register himself with the government watchdog, the Federal Commission for the Protection against Sanitary Risk (COFEPRIS).

"At that time, they had no frame of reference for this type of thing," he recalls. "At one point, they asked me for my clients' names and contacts, to make sure everything was above board. I couldn't do that. Their discretion is primordial for me. When you win their trust, you have to respect it through attention to detail and some sort of 'professional warmth', if that can be said to exist.

"The client is never in their ideal circumstances when I meet them, of course. That's why I read a lot of sociology and psychoanalysis - Durkheim on suicide, Freud on mourning and melancholy - to be sure I'm treating them correctly. The interaction I have with them is very short, and very delicate. I require logistical information, but they're vulnerable, so you have to be very human about asking the questions. I pay a lot of attention to this, so that's why I get a lot of word-of-mouth business.

"I arrive, I work, and I disappear."

'A true liberation of the scene'

Donovan becomes animated when discussing the results of his cleaning jobs, sitting more upright in his seat, gesturing, the cadences of genuine passion audible in his voice.

"The relatives look so tired, so sad when they are telling me what they need. The relief is just as visible when I'm finished. Our work is the last phase in their grieving process, coming after the collection of the body and the organisation of the funeral. The police call it 'liberating the scene' when the investigation is finished and the site of the event is open to access again, but the real liberation happens when I'm finished cleaning.

"I watch the families come in and breathe the air without the emotional shock of reliving that initial trauma. It's gone. The scene has been liberated. Certain events leave an emotional trace in the air, something like atoms of trauma. After one multiple-homicide in Colonia del Valle [a middle-class neighbourhood of Mexico City], for instance, there was a vapour in the air, the impotence and fear of the victims, the fury of the aggressor, the pain of everybody involved. At the end of the job, that wasn't there anymore. That's a true liberation of the scene."

We leave the diner, Donovan driving us around the empty streets of Mexico City at midnight. The sound of the motor undulated with the bass-lines of the traditional acoustic bolero songs playing on the radio.

"I prefer the city by night," he says, relaxing in his driver's seat, the solitude of his car seeming to make him more comfortable than the forced sociability of the diner. "I hardly go out during the day. The noise, the traffic, the quantity of people all around - it all makes me a bit claustrophobic," he says.

"The night is quiet and empty, and I can drive for miles and miles without interruption. I enter into a trance. Everything dissolves."

This same trance-like state descends upon him during his cleaning work. "My emotions freeze. I don't notice the time pass: There is too much to do," he explains.

"First, I scrape off the dried residues, then I add formulas to kill bacteria. If there is fauna or fungal formations of some sort, I dissolve these, too, before disinfecting the trash and contaminated objects on-site. Then I clean and clean until the room looks like new. Everything there seems brighter afterwards, as if there were more light coming into the room. It's a beautiful moment."

'A cold city'

When asked about the emotional impact of his work, he flexes his fingers on the steering wheel and answers with a heavy sigh.

"Look, the hard thing isn't the work: It's not having work," he explains, shifting gear and taking a route through the Blade Runner landscape of the capital's business district, all cold neons, gleaming skyscrapers and pulsing emptiness.

"In the 16 years that I've been cleaning full-time, I've seen this city and country get so cold. I've cleaned up after bodies forgotten inside residential buildings for weeks, sometimes months. The neighbours tell me that they only realise this person existed when the smell of their corpse begins to spread through the walls. Can you imagine?

"The indifference we live in gets stronger by the day, it seems like. I have to clean up more suicides than I used to, and more of these are about money issues. In the past, it was mostly people who couldn't live their sexuality openly."

Donovan has his own money issues. "I always have fears of falling into financial problems. When I'm between jobs, I think about this so much. Without work, I feel impotent," he says.

Government surveys indicate that almost half of the Mexican population live in some degree of poverty, while the OECD ranks Mexico 14th in the world for its levels of personal debt.

The impending privatisation of the country's vast onshore oil reserves has not brought the expected economic stimulus , while the national currency continues its steep freefall against the dollar.

"It's affected us directly," says Donovan , turning off the intestinal coil of the Periferico highway towards the city's main boulevard, Avenida Reforma.

"Straight zeros in the bank account after Christmas. We didn't receive a single cleaning. It's always like this, but the situation lasted longer this year. I became very stressed. Some work to live, others live to work. I do both."

Finding solace with the angel

The city's iconic Angel of Independence monument is lit up as the car slows to a halt on the empty roundabout. We get out.

"Cleanings are a ritual for me," Donovan explains. "They permit me to structure the chaos. I put on Tristan and Isolde by Wagner while I clean, because I genuinely feel something like heroic when I tape myself into the suit and put on my ventilator.

"[But] for those months [without work], I had no idea what to do with myself. I tried to remember other cleanings, because these excite me. I had to keep my cool, stay in the moment, same as I do when working. I had my Bach and Black Sabbath albums, the self-defence classes that I teach part time, but only spending time with my wife and daughter made me feel better. My daughter's six: too young to be upset by this work. She tells her classmates 'Daddy cleans dead people', and that's fine, but as she gets older, I want to protect her a bit from the more upsetting aspects of the job."

Donovan leans against the car and looks up at the Angel monument. The occasional wail of a siren cuts the night, but other than that, there is silence.

"I would love to change the whole country, but cleaning is the only contribution I can bring to the little part of the world that's under my control," he says.

"Corruption and impunity allow violence to continue in Mexico. Taking care of the dead, of the forgotten, is part of the solution. I'm almost always happy with my job, but there was one case that really disturbed me."

He lights a cigarette and takes a puff. "Even without the bodies, I can read what happened, see it again as in a film. In the stains, I saw people running, their faces, the knife coming down, the patterns that indicate attempts at self-defence.

"But I have my rituals for containing all that," he continues. "I drive here, for example. If I come back home and I'm stressed, my wife tells me it's because I didn't go to the Angel. She's always right."

'I have to be a model for the ones who come after me'

The bolero songs can be heard through the open car door: Los Panchos, Los Tres Diamantes, Los Dandys. The weak AM radio signal seems to cross the distance between the time they were recorded and the present day.

"I love this station," Donovan says with a contented sigh, exhaling a long garland of smoke. "There's no news. I can pretend [to be] in a safer, more innocent moment.

"I had a phase where I liked to drive all night out to small towns in adjacent states. I'd go to a cantina or a restaurant and get people to tell me their stories of the '50s, '60s, '70s ... all those lost moments. At that time, everything was safer, and their memories seemed to me to be even safer. Now, I wouldn't dare to take that kind of trip: It's all too dangerous."

He watches the arc of his flicked cigarette butt before continuing. "But I'm lucky. I love my family. My life with them is a space of safety, of innocence.