The second paper, published in 2018, discusses how certain organophosphorus compounds can induce apoptosis, particularly in neurons and myocardial cells, a process not reliant on cholinesterase inhibition.

The third article is a review summarizing the non-cholinergic toxic mechanisms of organophosphorus compounds discovered over the past decade, including oxidative stress, mitochondrial dysfunction, and neuroinflammatory responses.

Zhaxi grew more alarmed as he read, for if these articles were true, the clinical manifestations of organophosphorus poisoning are far more complex than written in clinical and forensic medicine textbooks. Symptoms like pupil constriction, salivation, and sweating might just be the tip of the iceberg. Beneath the surface, more unseen damage occurs, cells die, mitochondria collapse, and the nervous system slowly gets eroded.

He printed out these papers, highlighted the key points with a marker, and drew a new mind map in his notebook. He categorized organophosphorus poisoning into two types: typical poisoning and atypical poisoning. Typical poisoning primarily involves cholinergic crises, while atypical poisoning leads to cell damage, making its clinical manifestations more obscure and difficult to diagnose.

He recalled Professor Zhou’s death: pupil constriction was a typical symptom, but there was no salivation, sweating, or pulmonary edema. Could this be a case of atypical poisoning, one where cell damage predominates and cholinergic symptoms are not prominent?

He wrote this idea down and placed a question mark beside it.

At eight o’clock in the evening, Dr. Yuan returned Yang Ping’s call.

The chemical residue analysis results of the deceased’s clothing were out. No organophosphorus residue was found on the coat, shirt, or pants. However, trace organic solvent residues were detected on the shirt cuff near the wrist, composed of toluene and xylene. The levels were very low, insufficient to cause poisoning but suggested contact with organic solvents.

For a chemistry professor who regularly handles chemical agents, having some organic solvent on his cuffs is quite normal.

Yang Ping paused for a while and asked, "What about the tea? The book?"

"Trace organophosphorus was detected in the tea; the concentration was extremely low, about 0.5 ppm. Even drinking a whole cup wouldn’t be enough to cause poisoning. No organophosphorus was found on the book. As for the tea having trace organophosphorus, sometimes pesticide residue in market tea leaves is not uncommon."

Yang Ping’s brow furrowed further: "Organophosphorus was detected in the tea, but the concentration was not lethal. Then what caused his death?"

Dr. Yuan said, "We are continuing with toxicological screening and performing more comprehensive analyses, including rare toxin screening, but this process takes time."

After hanging up, Yang Ping sat and pondered for a while before passing the news to Zhaxi.

"Zhaxi, what do you think?"

Zhaxi thought for a moment and replied, "The concentration of organophosphorus detected in the tea is very low, not like poisoning. The cuffs showed toluene and xylene, indicating possible contact with organic solvents, but these factors don’t explain his death."

Yang Ping nodded, "So, we must be missing something."

He mentally added the clues obtained today:

Source: No open organophosphorus reagents at home or in the laboratory; cuffs showed toluene and xylene.

Route: The concentration of organophosphorus in the tea is too low to be the lethal pathway.

Dosage: Four months ago, cholinesterase was 2100, insufficient to be lethal.

Pigmentation: Symmetrical distribution at the fingertips suggests long-term local contact.

Tea organophosphorus: Concentration 0.5 ppm, not lethal.

Clothing cuffs: Detected toluene and xylene.

He suddenly asked a question that Zhaxi had not considered at all, "Zhaxi, do you know what toluene and xylene are used for?"

Zhaxi thought for a moment, "Organic solvents, used to dissolve... certain substances?"

Yang Ping nodded, "Yes, toluene and xylene are common organic solvents, particularly in chemistry labs, used to dissolve compounds that don’t dissolve in water. If a compound doesn’t dissolve in water, how would you get it absorbed through the skin?"

Zhaxi froze for a moment, then suddenly had a thought: "Dissolve it in organic solvents, make a solution?"

Yang Ping looked at him, his gaze becoming sharper, "Yes, many organophosphorus compounds are fat-soluble, insoluble in water but soluble in organic solvents like toluene and xylene. If you dissolve organophosphorus compounds in toluene and apply it on the skin, what would happen?"

Zhaxi’s mind raced, "Organic solvents can disrupt the skin barrier function, carrying organophosphorus through it. So, even though organophosphorus itself has low skin absorption, with the help of organic solvents, absorption rates would greatly increase."

Yang Ping nodded, "Yes, and organic solvents themselves are toxic. Toluene and xylene can cause neurological inhibition, arrhythmias, and respiratory depression. If a person were to contact both organophosphorus and organic solvents simultaneously, the toxic effects could be synergistically enhanced."

Zhaxi’s heartbeat accelerated. He suddenly considered a possibility that someone dissolved organophosphorus in organic solvents to make a solution. Then, by some method, it was made so that Professor Zhou had prolonged contact with this solution, particularly on his fingertips, where the skin is thinner, the blood vessels are abundant, and absorption is high. Prolonged exposure led to chronic poisoning, pigmentation, and continuous cholinesterase decline. Then at some point, a larger dose exposure caused acute poisoning and death.

And this "certain method" might just be the books.

Zhaxi blurted, "Those books!"

Yang Ping said nothing.

Zhaxi spoke faster, "Those books are kept so well, they’re not meant for reading. If someone had applied something on the pages so every time he flipped them, his fingertips would contact..."

Yang Ping interrupted him, "Do you have evidence?"

Zhaxi froze.

Yang Ping said, "Your hypothesis is logically sound. But there is no evidence, no residue detection results on the book pages, no report of organophosphorus detected in the fingerprints of the deceased—all are just speculations."

Zhaxi faced the phone, silent for a while, his curiosity growing stronger.

Yang Ping gently said, "Your thinking is correct, but solving cases is like diagnosing illnesses, you can’t rush. Step by step, each step needs evidence. Right now, what’s the key evidence we lack?"

Zhaxi thought for a moment and said, "Residue on the book pages. If those books were indeed treated, there should still be residue on them."

Yang Ping nodded, "Right, and the deceased’s fingerprints, if he frequently touched contaminated pages, his fingerprints should show organophosphorus or organic solvent residues."

He picked up the phone, dialed Dr. Yuan’s number, "Dr. Yuan, sorry to bother you, we have some thoughts to discuss. All the books in the deceased’s study, especially the frequently used ones, have they been tested for surface residue? Focus on organophosphorus, toluene, and xylene. Has a chemical residue analysis been performed on the deceased’s fingerprints?"

Dr. Yuan sounded surprised, "No, we didn’t consider these, but we can perform these tests now."

After hanging up, Yang Ping told Zhaxi, "Now, we wait for the results."

At night, Zhaxi lay in bed, his mind repeatedly dwelling on those books. He imagined Professor Zhou sitting at the desk, opening a book, his fingertips gliding over the pages. Once, twice, three times. A day, two days, three days. Six months, one hundred and eighty days. Each time, a little bit of organophosphorus solution seeped into his fingertip skin, entered the bloodstream, slowly eroding his nervous system, mitochondria, and cells.

This was a chronic, silent murder. The killer didn’t need to approach him, poison his food, or install any device in his room. All it took were a few books, a few bottles of reagents, and enough patience and deep knowledge of toxicology.

And this killer was likely someone close to Professor Zhou.

Zhaxi shuddered.

He checked the time, eleven-thirty. He remembered Yang Ping’s orders, turned off the light, and closed his eyes, but he couldn’t sleep. In his mind, those books flipped page by page, making a rustling sound.