Environmental Diseases — Smoking, UV Exposure, Asbestos and Lifestyle Factors

Q1.1 — Trend description

Male lung cancer mortality rose from approximately 55 per 100,000 in 1975, peaked at approximately 75–80 per 100,000 around 1985–1990, and has since declined to approximately 42 per 100,000 by 2020. The overall trend is an inverted-U (rise then sustained decline). Accept any plausible peak estimate of 70–82 in the 1985–1992 period. [2 marks: 1 for describing rise-then-fall, 1 for plausible peak estimate with year]

Q1.2 — Latency period explanation

Lung cancer has a latency period of 20–30 years between initial exposure and clinical disease. Male smoking peaked around 1945, so the cohort of men who smoked most heavily in the 1940s and 1950s began to develop lung cancer in the 1960s–1970s, with the mortality peak occurring 35–45 years after peak smoking prevalence [1]. This latency reflects the time needed for multiple tobacco carcinogen-induced mutations to accumulate in a single bronchial cell line to produce invasive lung cancer [1].

Q1.3 — Male vs female pattern

Accept any two of: (i) Female smoking rates peaked approximately 30 years later than male rates (around 1975 vs 1945), so the female lung cancer mortality curve lags the male curve by approximately 30 years — meaning the plateau in female rates around 2005 reflects female smoking patterns from the 1970s–1980s, and the decline has not yet been as steep because the time since peak is shorter [1]. (ii) Continued moderate smoking rates among women have maintained exposure levels, preventing the sharp decline seen in men whose rates fell earlier and more steeply [1]. (iii) Improvements in treatment (surgical resection, targeted therapies, immunotherapy) have modestly improved survival, which somewhat decouples mortality trends from incidence [1].

Q1.4 — Evaluate the 1:1 claim

The claim overstates the relationship. The dose-response relationship shows that lung cancer risk increases with cumulative tobacco exposure (measured in pack-years) — more smoking = higher risk, not a guaranteed outcome. Lung cancer is a multifactorial disease: not every heavy smoker develops lung cancer, and approximately 15% of lung cancers occur in non-smokers. Genetic susceptibility (e.g. DNA repair enzyme efficiency), other environmental exposures (radon, air pollution), and random variation in which cells accumulate sufficient mutations all modify the individual outcome. [2 marks: 1 for correctly applying dose-response to reject 1:1; 1 for multifactorial / probabilistic reasoning]

Q2 — Cause-and-effect chain

Row 1 effect: Macrophages are recruited to attempt to phagocytose and clear the foreign fibres.

Row 2 effect: The macrophages release reactive oxygen species (ROS) and inflammatory cytokines in a chronic, unsuccessful attempt to degrade the indestructible fibres (frustrated phagocytosis).

Row 3 effect: ROS directly damage the DNA of neighbouring mesothelial cells, causing strand breaks and oxidative base modifications.

Row 4 effect: Loss of tumour suppressor function removes cell cycle checkpoints; the affected mesothelial cells can now divide without normal regulation.

Row 5 effect: Uncontrolled mesothelial cell division produces a tumour mass in the pleural lining.

Overall outcome: Malignant mesothelioma develops, typically 20–50 years after initial asbestos exposure, due to the cumulative physical (not chemical) carcinogenic effect of persistent asbestos fibres on mesothelial cells.

Q3.1 — Additive prediction vs observed

Additive prediction for Group 4: asbestos alone (5×) + smoking alone (10×) = 15× baseline. Observed: 50–90× baseline. Because the observed risk (50–90×) is far greater than the additive prediction (15×), the interaction is classified as synergistic (supra-additive). [1 mark for correct additive calculation of ~15×; 1 mark for classifying as synergistic]

Q3.2 — Biological reasons for synergy

Accept any two of: (i) Asbestos fibres impair mucociliary clearance of the airways, increasing the residence time of tobacco carcinogens in bronchial tissue, effectively increasing the tobacco dose [1]. (ii) Both exposures converge on the same oncogenic pathway in bronchial epithelial cells: asbestos-driven ROS damages DNA in the same cells that tobacco carcinogens also target, creating a combined mutational load on the same cell population [1]. (iii) Asbestos-driven chronic inflammation creates a pro-proliferative and pro-mutagenic microenvironment (more cell division = more opportunities for tobacco-induced mutations to be replicated and propagated) [1].

Q3.3 — Dose-response prediction

The dose-response relationship predicts that an asbestos worker who smokes more cigarettes per day will have a higher cumulative carcinogen exposure, which increases the probability that sufficient mutations accumulate in a bronchial cell to produce lung cancer. The data support this: Group 4 (asbestos + heavy smoking) already shows 50–90× the baseline risk; heavier smoking further increases the mutational load from tobacco carcinogens, pushing risk even higher. [2 marks: 1 for applying dose-response reasoning; 1 for connecting to the data]

Q4.1 — Wittenoom latency explanation

Asbestos fibres are biopersistent — they are chemically inert and cannot be degraded by the body. Once inhaled, they remain lodged in the pleural lining indefinitely [1]. Asbestos-related mesothelioma has a latency period of 20–50 years from first exposure to clinical diagnosis [1]. Workers exposed in 1966 (when the mine closed) would enter the 20–50 year latency window between 1986 and 2016. Many individuals exposed between 1943 and 1966 are only now reaching the upper end of this window (2016–2022+), so new diagnoses continue to emerge decades after exposure ceased [1].

Q4.2 — Prediction for next 20 years

The rate of new diagnoses among directly exposed Wittenoom workers and residents is likely to decline over the next 20 years. The original exposure cohort is ageing and decreasing in size, and no new asbestos exposure is occurring since 2003 [1]. The dose-response relationship and latency window predict that as the 50-year upper latency limit is exceeded for those exposed in the earliest period (1943), the pool of individuals still within the at-risk latency window will shrink. However, some cases may continue to emerge from individuals who were exposed to low-level contamination in the surrounding area (e.g. from disturbed building materials), sustaining a small number of cases beyond the primary cohort [1].

Q5 — Epigenetics diagram critique

5.1 Error 1 — The diagram labels the process as “DNA mutation.” This is wrong: DNA methylation by tobacco smoke is an epigenetic change, not a mutation. The DNA nucleotide sequence of CDKN2A is not altered — a methyl group is added to cytosine, but the cytosine nucleotide remains in place. Correction: relabel the process as “DNA methylation (epigenetic change)” and note that the nucleotide sequence is unchanged.

5.2 Error 2 — Methyl groups are shown attaching to adenine (A) bases. This is wrong: DNA methylation in the context of gene silencing occurs at cytosine (C) bases within CpG dinucleotide sites. Correction: redraw methyl groups attaching to cytosine (C) bases in the CpG sites of the promoter region.

5.3 Error 3 — The result is labelled “CDKN2A gene is activated.” This is wrong: methylation of the CDKN2A promoter silences the gene by preventing transcription factor binding. Correction: change the label to “CDKN2A gene is silenced / transcription repressed” and add that p16 protein is not produced, removing a cell cycle checkpoint.