Toroku arsenic disease

Toroku arsenic disease is chronic arsenic poisoning that affected residents and workers around an arsenic refinery at the Toroku mine in Takachiho, Miyazaki Prefecture, Japan, where airborne and waterborne arsenic contaminated a narrow valley during two periods of production in the twentieth century

The disease is characterised by diffuse or macular pigmentation, raindrop-shaped depigmentation, palmar and plantar keratoses, nasal septal perforation and peripheral neuropathy, with skin cancer, lung cancer and urinary tract cancer occurring as late complications

Toroku is one of Japan's pollution-related diseases, and its history illustrates how environmental exposure, delayed recognition and legal compensation interact in the epidemiology of chronic toxic disease

Terminology and classification

• Toroku arsenic disease is also called Toroku arsenic pollution or Toroku hiso kogai in Japanese, and it is classified as a pollution-related chronic arsenicosis rather than as an infectious or genetic disease
• Chronic arsenicosis is the general category, defined by prolonged exposure to inorganic arsenic with characteristic skin lesions and an increased risk of internal cancers and neuropathy
• The Toroku outbreak is distinguished from acute arsenic poisoning, which causes vomiting, diarrhoea, shock and arrhythmia within hours, and from occupational arsenic exposure without environmental contamination of a community
• ICD-10 codes arsenic toxicity under T57.0, MeSH uses D020260 for arsenic poisoning, and Wikidata uses Q7826225, while the cancers that complicate the disease are coded separately by site
• The main named differentials are other Japanese arsenic pollution sites such as Matsuo, Nishikawa and Nakajo, non-arsenic keratotic skin disease, Bowen's disease without arsenic exposure and other causes of peripheral neuropathy
• Because the exposure ended decades ago, the disease is now encountered mainly as a late-effect and compensation category rather than as a new environmental emergency, although contaminated soil and water remain a remediation concern

History

• Arsenic production at Toroku occurred between 1920 and 1941 and again between 1955 and 1962, and the refinery released arsenic-containing smoke that remained trapped in the deep V-shaped valley
• A veterinarian recorded the deaths of horses and cattle in 1925 that were possibly related to pollution, an early signal that the environmental exposure was affecting animals as well as people
• In 1967 the mining rights passed from a small factory to Sumitomo Metal Mining, and the change in ownership later became central to the legal case brought by affected residents
• Tsurue Sato sought advice at a Legal Affairs Bureau branch on 8 December 1970, and teacher Masa Saito published the health damage on 13 November 1971, bringing the outbreak to public attention
• Kumamoto University researchers diagnosed seven patients with pollution-related chronic arsenic poisoning in a 1973 report and expanded the series to 48 patients by 1976, while Hotta and colleagues studied the environmental and clinical features
• The residents' lawsuit began in 1975 and ended about fifteen years later in a compromise, and annual medical examinations of residents have continued since 1974, providing one of the longest follow-up records of chronic arsenicosis

Epidemiology

• The exposed population was small and localised, and the reported patient series grew from seven in 1973 to 48 in 1976, with additional residents identified through annual examinations after exposure had ceased
• Both refinery workers and residents were affected, because smoke and contaminated water spread beyond the plant and because the valley topography concentrated the emissions
• Skin lesions were the most frequent finding, followed by peripheral neuropathy and hearing loss, and cancers of the skin, lung and urinary tract appeared as late complications in long-term follow-up
• Japan has several arsenic pollution sites, including Matsuo in Shimane Prefecture and Nishikawa and Nakajo in Niigata Prefecture, and comparisons between them helped establish dose-response relationships for arsenic-related cancer
• The Toroku cohort is small, so incidence rates for individual cancers are imprecise, but the excess of skin and lung cancer is consistent with the known carcinogenicity of inorganic arsenic
• Exposure was both inhalational and ingestional, through contaminated air, soil, rice and drinking water, and the relative contribution of each route is difficult to reconstruct retrospectively
• Because blood arsenic was not measured during the exposure period, later investigators defined many residents as having probable rather than confirmed arsenic intoxication, based on exposure history and clinical findings
• The social burden includes chronic disability, cancer treatment, compensation claims and community stigma, and these consequences persist long after the environmental source has been controlled

Pathophysiology

• Inorganic arsenic exists as trivalent arsenite and pentavalent arsenate, and the trivalent form reacts with sulfhydryl groups in enzymes such as pyruvate dehydrogenase, disrupting cellular respiration and energy metabolism
• Arsenic generates reactive oxygen species, depletes glutathione and damages DNA, and it inhibits DNA repair, which explains both its direct toxicity and its activity as a co-carcinogen
• Chronic exposure produces hyperpigmentation, hypopigmentation and hyperkeratosis, and these skin lesions are the clinical hallmarks of arsenicosis and the setting in which Bowen's disease and squamous cell carcinoma arise
• Arsenic is an established human carcinogen for skin, lung and urinary bladder, and the mechanism involves chromosomal aberrations, oxidative stress and epigenetic changes rather than direct point mutation
• Peripheral neuropathy in arsenicosis is predominantly axonal and sensory, producing distal numbness, burning and weakness, and hearing loss may result from cochlear or retrocochlear damage
• Arsenic also affects blood vessels and the heart, and chronic exposure is associated with hypertension, ischaemic heart disease and peripheral vascular disease, although the classic Blackfoot disease phenotype is most associated with arsenic in drinking water in Taiwan

Clinical features

• Diffuse or macular pigmentation on the trunk and extremities, together with raindrop-shaped punctate depigmentation, is the classic dermatological picture and is often the first recognized sign
• Palmar and plantar keratoses are firm, yellowish plaques that may fissure and become painful, and they are the precursor lesions for arsenic-related squamous cell carcinoma
• Bowen's disease, squamous cell carcinoma and basal cell carcinoma arise in areas of chronic arsenical dermatosis, and multiple lesions may occur over years
• Nasal septal perforation, chronic rhinitis and anosmia reflect direct irritant and vascular effects of inhaled arsenic, and they were used as diagnostic criteria in the Toroku compensation system
• Peripheral neuropathy presents with symmetrical distal numbness, paraesthesia, burning pain and weakness, and in severe cases distal muscle wasting and gait disturbance
• Long-term survivors have an increased risk of lung cancer and cancers of the bladder and kidney, and they also report fatigue, hearing loss, skin disfigurement and psychological distress related to compensation and stigma

Diagnosis

• The diagnosis rests on a documented history of exposure to arsenic-contaminated air, water, soil or food, combined with characteristic skin lesions, neuropathy or cancer at a compatible latency
• Urinary arsenic speciation distinguishes inorganic arsenic and its metabolites from less toxic organic arsenicals found in seafood, and it is the preferred test for recent exposure
• Hair and nail arsenic can document past exposure because arsenic binds keratin, but external contamination and individual growth rates limit interpretation
• Skin biopsy of keratotic or pigmented lesions identifies Bowen's disease or invasive carcinoma, and any non-healing ulcer or new nodule in an arsenic-exposed patient should be biopsied
• Cancer surveillance includes chest imaging or low-dose computed tomography, urinalysis and cystoscopy when indicated, and dermatological examination at regular intervals
• The differential includes other causes of palmoplantar keratoderma, pigmentary disorders, chronic neuropathy and lung or bladder cancer unrelated to arsenic, and exposure history is the key discriminator

Management

• The first principle is to stop exposure and to remediate contaminated soil, water and food sources, because further exposure worsens both cancer risk and neurological outcome
• Chelating agents such as dimercaptopropane sulfonate or dimercaptosuccinic acid are used for acute or severe arsenic poisoning, but their benefit in chronic arsenicosis with established skin and neurological damage is unproven
• Skin cancers and Bowen's disease are treated by surgical excision, curettage, cryotherapy or topical therapy depending on size, site and number, and patients need long-term dermatological surveillance for new lesions
• Peripheral neuropathy is managed with neuropathic pain medication, physiotherapy, ankle-foot orthoses and fall prevention, and hearing loss with hearing aids and communication support
• Lung and bladder cancers are treated according to stage and histology with surgery, radiotherapy, chemotherapy or immunotherapy, and early detection through surveillance improves the chance of curative treatment
• Nutritional support, smoking cessation and control of hypertension and diabetes reduce competing risks, and rehabilitation addresses gait, hand function and activities of daily living
• Compensation and social support are part of care, because many patients face long-term medical costs, occupational loss and psychological distress, and legal recognition can affect access to health services
• Public health follow-up includes continued health examinations of exposed residents, cancer screening and monitoring of remediation, because the latency of arsenic-related cancer can exceed several decades

Research and controversies

• The 2018 clinical and electrophysiological study of surviving Toroku residents found sensory neuropathy and hearing loss decades after exposure, supporting the view that arsenic neurotoxicity can be permanent
• Whether chronic low-level arsenic exposure causes the same cancer spectrum as high-dose exposure is uncertain, and studies of contaminated drinking water worldwide continue to inform dose-response models
• The role of chelation in chronic arsenicosis is controversial, because removing arsenic from the body may not reverse established skin, nerve or vascular damage and may redistribute arsenic to other tissues
• Biomarkers that predict which exposed individuals will develop cancer are lacking, and research on epigenetic marks, telomere length and immune profiles may allow better targeting of surveillance
• The Toroku case raises unresolved questions about compensation for pollution-related disease, including how to attribute cancer to past exposure, how to value non-economic harm and how to sustain care when the original industry has gone