Which of the following pollutants can cause cancer in humans?

Concept: A carcinogen is an agent that can initiate or promote cancer, typically by damaging cellular DNA or by driving sustained abnormal cell division. The WHO's International Agency for Research on Cancer (IARC) integrates human epidemiology, animal bioassays and mechanistic evidence to place an agent in Group 1 (carcinogenic to humans), Group 2A (probably carcinogenic), Group 2B (possibly carcinogenic) or Group 3 (not classifiable); Group 1 spans very different pollutants, from asbestos, benzene, arsenic and hexavalent chromium to outdoor air pollution and certain pesticides. When an item asks which pollutant can cause cancer, the discriminator is therefore the level of evidence: which family contains agents established as human carcinogens, as against families whose recognised human injury is non-cancerous and whose cancer evidence rises only to probable or possible.

Application: Pesticides are a heterogeneous family rather than a single chemical, and the evidence attaches to particular active ingredients and exposure circumstances; even so, several members are established human carcinogens. IARC classifies lindane, an organochlorine insecticide, and the wood preservative pentachlorophenol in Group 1, and occupational exposure during the spraying and application of non-arsenical insecticides in Group 2A; cohort studies of farm and spray workers link long-term exposure to non-Hodgkin lymphoma, leukaemia and prostate cancer. Persistent organochlorines such as DDT and lindane compound this, because they resist breakdown, biomagnify up the food chain and are stored in body fat, so the internal dose continues for decades after application. Pesticides are therefore the family in this set that contains Group 1 human carcinogens.

Cross-check — compare the evidence standing of the other pollutants offered here:

• Mercury: the recognised human injury is neurological. Sediment bacteria methylate it, methylmercury crosses the blood–brain barrier, and the classic mass poisoning is Minamata disease (tremor, ataxia, congenital deformity). On cancer, IARC reaches only Group 2B for methylmercury and Group 3 for metallic and inorganic mercury — suspicion rather than established carcinogenicity.

• Lead: the recognised human injury is neurotoxic and haematotoxic. It substitutes for calcium, inhibits haem synthesis (anaemia) and permanently lowers cognitive development in children. On cancer, inorganic lead compounds reach Group 2A — probable, and therefore still short of established.

• Ozone (O3): at ground level a secondary photochemical pollutant and airway irritant that aggravates asthma and reduces lung function, and it is not classified as a human carcinogen. In the stratosphere ozone is the protective shield, and it is the UV-B admitted by its depletion — not the ozone itself — that causes skin cancer.

Result: Pesticides. Read at its loosest, "can cause" would also admit the lower-grade suspicion attached to lead (Group 2A) and methylmercury (Group 2B); the discriminator the item intends is established human carcinogenicity, which among these four only the pesticide family reaches, and the official UGC NET December 2015 Paper I answer key records Pesticides as the answer.