|Fig. 1: A Map of the Navajo Nation. (Source: Wikimedia Commons)|
During World War II, the United States quest to develop nuclear weapons led to a surge in demand for uranium. In 1948, the US Atomic Energy Commission (AEC) declared that it would purchase all uranium ore mined within the United States.  Uranium mines began to emerge throughout the Southwest, particularly on the Colorado Plateau. Several mines were established in the Navajo Nation, a sovereign Native American state spanning Utah, New Mexico, and Arizona with a population of over 250,000 (see Fig. 1).  Due to a lack of other economic opportunities on the reservation, the Navajo people initially welcomed the presence of uranium mines on their lands. Navajo men began working in the mines, unaware of the health hazards associated with uranium mining.
In mines, uranium ore decays into radon gas, which itself breaks down into radioactive radon daughters, including Po-218, Bi-214, and Po-214, which have a short half-life of 30 minutes. The radon daughters are absorbed by airborne dust particles and inhaled by workers in the mine. Radioactive species (particularly Po-218 and Po-214, which deliver the majority of oncogenic radiation) accumulate along the pulmonary epithelium where emitted α-particles introduce single and double-stranded breaks to DNA, potentially leading to diseases such as lung cancer. [3,4]
Epidemiologists assert that a relationship between cumulative radon exposure and an increased risk of radon death exists; multiple studies have analyzed several cohorts across various environments and time periods. In one epidemiological study of over 5,000 French miners between 1946 and 1997, researchers used an excessive relative risk model to calculate the exposure-risk relationship between radon exposure and lung cancer, and determined that it was significant (ERR per 100 WLM = 0.58, p < 0.01). Period of exposure was the most significant modifier of this exposure-risk relationship (p = 0.001).  As a reference, one working level is defined as "a concentration of radon decay products that will release 1.3 million electron volts per second per liter of air," or 100 pCi/L in air. A working level month is equivalent to spending 170 hours exposed to one working level. [3,6] Additionally, it appeared that the relationship between relative risk of lung cancer and radon exposure was approximately linear. [5,7] Other studies suggest that there is a strong inverse dose-rate, or protraction enhancement pattern, in which individuals exposed to low-level amounts of radiation over a longer period of time might have a greater risk of lung cancer than an individual exposed to the same total amount of radiation over a short period of time.  When assessing the role of tobacco in lung-cancer related mortality amongst uranium miners, a study indicated that the excess risk for lung cancer after being exposed to radon, even after adjusting for smoking habits, remains significant (ERR = 0.85 per 100 WLM, 95% CI: 0.12-2.79%).  It should be noted, however, that epidemiological studies of lung cancer in uranium miners do not often take into account the affects of other potential carcinogens, such as gamma radiation. This may be a confounding variable that should be analyzed as well. 
The associations between uranium mining and lung cancer had been established since 1930, and ventilation had been a known solution for mitigating the risk of developing disease. However, the federal government did not create regulations mandating the adoption of preventive measures; rather, it simply encouraged mine operators to improve working conditions. Consequentially, many workers continued to work while being exposed to hazardous levels of radon. In fact, because the risk of radioactive materials such as uranium was not communicated to the Navajo, they allowed their livestock to drink from contaminated pools and built homes out of discarded material from the mines. 
By the 1960s, cases of lung cancer began to emerge on the Navajo Nation. Prior to uranium mining, the disease was rare on the reservation tobacco use outside of ceremonies was relatively uncommon.  In a survey of lung cancer in Navajo men between 1969 to 1982, 72% of Navajo lung cancer patients had once worked in a uranium mine. Another study of a cohort of Navajo uranium miners diagnosed with lung cancer, 62.5% were diagnosed with small cell lung cancer (SCLC).  While rarer than its counterpart non-small cell lung cancer, SCLC is far more aggressive. SCLCs have a high mitotic rate, metastasize easily, and lead to extensive necrosis, rendering treatment to be highly challenging.  The average amount of cumulative radon exposure for these workers was 1139.5 working level months (WLMs) over 11 years in the mines. Due to low levels of tobacco use across the reservation, the incidences of lung cancer have been largely attributed to radon exposure. In fact, the risk of developing lung cancer was approximately 20 to 30 times greater for Navajo men who were former uranium miners than Navajo men who had never worked in the mines. 
Today, the case of uranium mining and lung cancer in the Navajo Nation serves as an example of the egregious lack of care or concern for the safety of Native individuals. Safety precautions and ventilation should have been made available to workers in the mines as soon as the associations between lung cancer and uranium mining were established. Furthermore, more outreach should have been conducted to Native communities to educate individuals about their occupational hazards.
© Hannah Nguyen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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