Editing Openai/67ec19d7-5118-8001-9448-2258bb11399d (section)

==== Dose–Response at Low Exposure Levels: Is There a Threshold? ====

Current scientific consensus is that mesothelioma risk increases essentially linearly with cumulative asbestos exposure, with ''no clear threshold'' below which risk is zeropmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC1757769/#:~:text=OBJECTIVES%3A%20To%20estimate%20the%20importance,which%20has%20been%20often%20cited|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. A review of low-dose asbestos exposures concluded that “there is no evidence of a threshold level below which there is no risk of mesothelioma”pmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC1757769/#:~:text=OBJECTIVES%3A%20To%20estimate%20the%20importance,which%20has%20been%20often%20cited|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. Even very small exposures add a minute increment to risk. The reasoning is that inhaling even a single asbestos fiber carries a tiny chance of lodging in the mesothelial lining and eventually triggering cancer. Of course, the probability increases with the number of fibers inhaled (dose) and the time since exposure (latency). Mesothelioma risk tends to rise exponentially with time after exposure (after a latency of ~10 years), often peaking 30–50 years post-exposurencbi.nlm.nih.gov<ref>{{cite web|title=ncbi.nlm.nih.gov|url=https://www.ncbi.nlm.nih.gov/books/NBK597324/?report=printable#:~:text=Several%20studies%20%28e,to%20the%20following%20equation|publisher=ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. This is why even a one-off exposure can lead to mesothelioma decades later – the risk does not “reset to zero” after exposure ends, but instead the hazard persists and actually grows with time for the rest of the person’s lifencbi.nlm.nih.gov<ref>{{cite web|title=ncbi.nlm.nih.gov|url=https://www.ncbi.nlm.nih.gov/books/NBK597324/?report=printable#:~:text=Several%20studies%20%28e,to%20the%20following%20equation|publisher=ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>.

Regulatory agencies like the U.S. EPA model asbestos cancer risk as a function of cumulative exposure without a threshold. Ultra-low exposures still carry some risk, but it becomes vanishingly small. For example, EPA’s risk assessment estimated that a lifetime continuous exposure to 0.0001 fibers/mL of air (an extremely low concentration) would result in approximately 2 to 3 mesothelioma deaths per 100,000 people exposedncbi.nlm.nih.gov<ref>{{cite web|title=ncbi.nlm.nih.gov|url=https://www.ncbi.nlm.nih.gov/books/NBK597324/?report=printable#:~:text=encountered%20in%20the%20environment,as%20shown%20in%20%2011|publisher=ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>archive.cdc.gov<ref>{{cite web|title=archive.cdc.gov|url=https://archive.cdc.gov/www_atsdr_cdc_gov/csem/asbestos/who_is_at_risk.html#:~:text=for%20asbestos%2C%20even%20with%20aggressive,dust%20would%20present%20little%20risk|publisher=archive.cdc.gov|access-date=2025-11-28}}</ref>. In other words, even at 0.0001 f/mL (which is 1000 times below occupational limits), a very slight risk remains (on the order of 0.002% incidence over a lifetime)ncbi.nlm.nih.gov<ref>{{cite web|title=ncbi.nlm.nih.gov|url=https://www.ncbi.nlm.nih.gov/books/NBK597324/?report=printable#:~:text=encountered%20in%20the%20environment,as%20shown%20in%20%2011|publisher=ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. This illustrates the linear no-threshold assumption: cut the exposure in half and the risk is roughly halved; increase it and risk rises proportionally.

Some researchers have investigated whether very high doses might have a sub-linear dose-response (due to fiber overload, competing risks, etc.), but for pleural mesothelioma the data are consistent with an approximately linear increase in risk even down to low levelspmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC4853597/#:~:text=Results|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>pmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC4853597/#:~:text=Lifetime%20mesothelioma%20risk%20is%20approximately,and%20chrysotile%20%282|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. In a large British study, the lifetime mesothelioma risk was found to be about 0.02% per 1,000 amphibole fibers per gram of lung tissue – and this relationship held across a wide range of exposures from environmental levels up to heavily exposed workerspmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC4853597/#:~:text=Results|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. The authors noted the dose–response was approximately linear, confirming that lower doses produce lower (but non-zero) risk in proportionpmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC4853597/#:~:text=Lifetime%20mesothelioma%20risk%20is%20approximately,and%20chrysotile%20%282|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>pmc.ncbi.nlm.nih.gov<ref>{{cite web|title=pmc.ncbi.nlm.nih.gov|url=https://pmc.ncbi.nlm.nih.gov/articles/PMC4853597/#:~:text=Conclusions|publisher=pmc.ncbi.nlm.nih.gov|access-date=2025-11-28}}</ref>. Therefore, we must assume any amosite exposure contributes some risk, but the key is quantifying how much risk a one-off moderate exposure imparts, relative to other scenarios.