Two years ago, the future for college graduate Tanya Watters could hardly have been brighter.

She had just graduated with a first-class degree in Biochemistry with Immunology from Trinity College and by Christmas 2006, had been offered a PhD position in Molecular Immunology at the college.

It seemed the perfect time for a long-planned back-packing trip in South-East Asia with younger sister Michelle, but a diving accident has left her with Multiple Chemical Sensitivity (MCS), a chronic, debilitating illness.

Today, the 25-year-old gym-loving extrovert, who was on the cusp of a brilliant career, is a virtual prisoner in her own home in Ardara, Co Donegal, where even a whiff of perfume can send her to bed for hours.

"My life was really just beginning. I was starting to do things that I wanted to do. I had put so much effort into college. I loved my lab work. I had begun to pay off my loans and I was excited about travelling. And then this happened and everything changed in an instant," she explained.

Broke, in debt and unable to work, Tanya now rarely ventures beyond the confines of the family bungalow.

Out of doors, she wears a face mask with a carbon filter, enduring the stares of passers-by on her weekly outing to the health food shop in Donegal town to stock up on organic food.

Tanya has to constantly be on her guard for smells that go unnoticed by most people. The gas cooker, synthetic floor coverings, furnishings and curtains have been removed. The house has been cleared of all chemical detergents and cleaning agents. Perfumes and cosmetics have become taboo. Even reading a newspaper or a book can trigger symptoms, which range from dizziness and headaches, to shortness of breath, aching limbs, a sore throat and extreme fatigue.

Visitors who still call are warned in advance not to wear anything that could trigger a reaction in Tanya.

This once-energetic young woman tires easily and needs a lot of sleep. Her once-hectic social scene is now reduced to computer chat rooms and social networking sites.

"I don't have a life any more. I would actually rather have died at that stage. This is not a life. It's just an existence and there is the prospect that it might never end."

The nightmare began in Malaysia, six weeks into their three-month dream trip. The sisters had just completed an open-water scuba-diving course before travelling to the Parentian islands where they immediately signed up to go diving.

On the first dive, Tanya was unnerved after being attacked and badly stung by jellyfish. On her second dive, she became stuck in rocks before she ascended.

"We were at the safety stop and I got this really terrible pain in my head. I just remember thinking I have to go down to help myself equalise but then I just dropped a way down.

"The next thing I realised, it was all dark blue everywhere. I couldn't see anybody and I didn't know where I was. I started to swim up. I probably swam up pretty quickly the second time.

"I felt so disorientated and nauseous and dizzy and strange. It was like all of a sudden, I had the worst flu ever," she recalled.

Despite protracted symptoms, the dive school staff convinced her it could not be decompression illness because of the shallowness of the dive and it was seven days before she finally received Hyperbaric Oxygen Therapy (HBOT) in Bangkok.

She later learned that she had suffered an air embolism plus decompression illness, for which HBOT was necessary as quickly as possible to avoid a permanent brain injury.

Although she initially started to feel better after the HBOT treatment in Bangkok, she began developing new symptoms, the cause of which was later identified as oxygen toxicity.

Over the following three months Tanya continued HBOT treatment but her condition continued to deteriorate. By then, Tanya had been forced to abandon any thought of returning to Trinity.

"I thought I was never going to have a life again, pretty much, because I couldn't do anything and I just couldn't get any help. No-one could tell me what was going on.

"It was like I was transplanted into someone else's body and I just couldn't deal with it. It wasn't me. Even though I looked the same to people and I sounded the same, I wasn't."

A desperate Tanya finally found a doctor in New Orleans who diagnosed that she was suffering from oxygen toxicity. She relocated to the US for six months but almost on arrival she began noticing a chemical sensitivity that she had never experienced before.

"I couldn't tolerate exhaust fumes, gas cookers, perfume, air fresheners or cleaning products. The more I was exposed to chemicals, the more sensitised I would become and smaller amounts started to affect me.

"Looking back, even if I hadn't gone to New Orleans it would probably have happened anyway; maybe it would have taken longer."

Rat poison exposure sent Tanya plummeting to her worst, leaving her virtually bed-bound for two months. She finally decided that she had a reduced chance of chemical exposure if she returned to Donegal.

She returned to the family home in August where her distraught parents, Shaun and Mary, are struggling to come to terms with Tanya's illness.

"It is only since she came home that we truly understand Tanya's condition. It is like going through a mourning process. Tanya loved her career and she loved life. She should be out there enjoying this part of her life," said mum Mary, tears streaming down her face.

"Her sister Michelle is in England training to be a primary school teacher, but for Tanya, life has stopped."

They are in the midst of the costly process of converting her teenage brother's bedroom into a 'safe room' for Tanya, ridding it of furniture, floor and wall coverings, and replacing it with organic materials but even that will not be enough.

Tanya is struggling to find a GP or any experts on MCS in Ireland.

"I think MCS is trivialised so much because it is an invisible illness. None of this should have happened to me.

"I have lost everything because of it and so have many other people. I've lost the things I was passionate about -- my work, travel, exercise. I've had to let go of my dreams and ambitions.

"I've lost my freedom and independence, and I valued both of those things so much. I've lost my financial independence and my social life. It has also negatively affected almost every one of my relationships," she said.