Question: What did a quarter million construction workers reach their lifetime limits on
About the article "Chernobyl disaster", section "Debris removal".

Candidate passages:
The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris.
The reactor itself was covered with bags of sand, lead and boric acid dropped from helicopters: some 5,000 metric tons of material were dropped during the week that followed the accident.
At the time there was still fear that the reactor could re-enter a self-sustaining nuclear chain-reaction and explode again, and a new containment structure was planned to prevent rain entering and triggering such an explosion, and to prevent further release of radioactive material.
This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation.
By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.
A unique "clean up" medal was given to the workers.
Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area.
During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion.
These scientists manually collected cold fuel rods, but great heat was still emanating from the core.
Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes.
The scientists were exposed to high levels of radiation and radioactive dust.
After six months of investigation, in December 1986, they discovered with the help of a remote camera an intensely radioactive mass in the basement of Unit Four, more than two metres wide and weighing hundreds of tons, which they called "the elephant's foot" for its wrinkled appearance.
The mass was composed of sand, glass and a large amount of nuclear fuel that had escaped from the reactor.
The concrete beneath the reactor was steaming hot, and was breached by solidified lava and spectacular unknown crystalline forms termed chernobylite.
It was concluded that there was no further risk of explosion.
Generally the act requires parity of mental health benefits with medical and surgical benefits with respect to the application of aggregate lifetime and annual dollar limits under a group health planIt provides that employers retain discretion regarding the extent and scope of mental health benefits offered to workers and their families, including cost sharing, limits on numbers of visits or days of coverage, and requirements relating to medical necessity. 
In recommendations by the NAS in 1967, it was noted that radiation protection in human space flight is philosophically distinct from the protection practices of terrestrial workers because of the high-risk nature of space missions.
This report by the NAS from 1967 did not recommend "permissible doses" for space operations, noting the possibility that such limits may jeopardize the mission, but instead estimated what the likely effects would be for a given dose of radiation.
In 1970, the NAS Space Science Board recommended guidelines for career doses to be used by NASA for long-term mission design and human operations.
At that time, NASA employed only male astronauts and the typical age of astronauts was 30 to 40 years.
A "primary" reference risk" was proposed that was equal to the natural probability of cancer over a period of 20 years following the radiation exposure (using the period from 35 to 55 years of age); this was essentially a doubling dose.
The estimated doubling dose of 382 rem (3.82 Sv), which ignored dose-rate reduction factor, was rounded to 400 rem (4 Sv).
The NAS panel noted that its recommendations were not risk limits, but, rather, a reference risk, and that a higher risk could be considered for planetary missions or a lower level of risk for a possible space station mission.
Ancillary reference risks were described to consider monthly, annual, and career exposure patterns.
However, the NAS recommendations were implemented by NASA as dose limits that were used operationally for all missions until 1989.
At the time of the 1970 NAS report, the major risk from radiation was believed to be leukemia.
Since that time, the maturation of the data from the Japanese atomic-bomb survivors has led to estimates of higher levels of cancer risk for a given dose of radiation, including the observation that the risk of solid tumors following radiation exposure occurs with a higher probability than that of leukemias, although with a longer latency period before expression.
Figure 4-5 illustrates the changing estimates of cancer risks for an average adult worker since 1970.
Together with the maturation of the atomic-bomb data, reevaluation of the dosimetry of the atomic-bomb survivors, scientific assessments of the dose response models, and dose-rate dependencies have contributed to the large increase in risk estimate over this time period (1970 - 1997).
The possibility of future changes in risk estimates can, of course, not be safely predicted today; it is possible that such changes Normal 0 false false false EN-US X-NONE X-NONE could potentially impact NASA mission operations.
Thus, protection against uncertainties is an ancillary condition to the ALARA principle, suggesting that conservatism be exercised as workers approach dose limits.
By the early 1980s, several changes had occurred that led to the need for a new approach in defining dose limits for astronauts.
At that time, NASA requested that the NCRP reevaluate the dose limits that were to be used for LEO operations.
Considerations included the increases in estimates of radiation-induced cancer risks, the criteria for risk limits, and the role of the evolving makeup of the astronaut population from male test pilots to a larger, diverse population of astronauts (~100), including mission specialists, female astronauts, and career astronauts of higher ages who often participate in several missions.
In 1989, NCRP Report No.
98 recommended age- and gender-dependent career dose limits using a 3% increase in cancer mortality as a common risk limit.
The limiting level of 3% excess cancer fatality risk was based on several criteria, including a comparison to dose limits for ground radiation workers and to rates of occupational death in less-safe industries.
It was noted that astronauts face many other risks, and that an overly large radiation risk was not justified.
It also should be noted that the average years of life loss from radiation-induced cancer death, which is about 15 years for workers over age 40 years and 20 years for workers between age 20 and 40 years, is less than that of other occupational injuries.
A comparison of radiation-induced cancer deaths to cancer fatalities in the U.S. population is also complex because of the smaller years of life loss in the general population, where most cancer deaths occur above 70 years.
In the 1990s, the additional follow-up and evaluation of atomic-bomb survivor data led to further increases in the estimated cancer risk for a given dose of radiation.
Recommendations from the NCRP, while keeping in the basic philosophy of risk limitation that had been in the earlier report, advocate significantly lower limits than those that were recommended in 1989.
Table 4-9 provides examples of career radiation limits for a career duration of 10 years, with the doses assumed to be spread evenly over the career.
The values from the previous report are also listed for comparison.
Both of these reports specify that these limits do not apply to Exploration missions because of the large uncertainties in predicting the risks of late effects from heavy ions.
The NCRP Report No.
132 noted that the use of comparisons to fatalities in the less-safe industries that were advocated by the NCRP in 1989 were no longer viable because of the large improvements that had been made in ground-based occupational safety.
Table 4-10, which shows an update to such a comparison, demonstrates that, indeed, the decreased rate of fatalities in the so-called less safe industries (e.g., mining, agriculture) would suggest a limit below the 3% fatality level today as compared to that in 1989.
The most recent reviews of the acceptable levels of radiation risk for LEO, including that provided during a 1996 NCPR symposium and the recent NCRP Report No.
132 on the LEO dose limits, instead advocate that comparisons to career dose limits for ground-based workers be used.
It is also widely held that the social and scientific benefits of space flight continue to provide justification for the 3% risk level for astronauts who are participating in LEO missions.
In comparison to the limits that have been set by NASA, the U.S. nuclear industry uses age-specific limits that are gender-averaged, which is of sufficient accuracy for the low doses received by nuclear workers.
Here career limits are set at a total dose-equivalent that is equal to the individual's age × 0.01 Sv.
It is measured by the NCRP that ground workers who reach their dose limits would have a lifetime risk of about 3%, but note the difference in dose values corresponding to the limit is due to differences in how the radiation doses are accumulated over the worker's career.
The short term (30-day and 1-year) dose limits set by NASA are several times higher than those of terrestrial workers because they are intended to prevent acute risks, while the annual dose limits of 50 mSv (5 rem), which are followed by U.S. terrestrial radiation workers, control the accumulation of career doses.
The exposures that are received by radiation workers in reactors, hospitals, etc.
rarely approach dose limits with the average annual exposure of 1 to 2 mSv, which is a factor of 25 below the annual exposure limit and significantly less than the average dose for a 6-month ISS mission (100 mSv).
Similarly, transcontinental pilots, although they are not characterized as radiation workers in the U.S., receive an annual exposure of about 1 to 5 mSv, and enjoy long careers without approaching the exposure limits that are recommended for terrestrial workers in the U.S..
Under these conditions, ground-based radiation workers are estimated to be well below the career limits, even if a 95% CL is applied.
As space missions have been relatively short duration in the past, thereby requiring minimal mitigation, the impact of dose limits when space programs actually approach such boundaries, including the application of the ALARA principle, has been unexplored. 
In 2008, a Human Rights Watch report described unsafe and unfair working conditions and failure on the part of the Chinese government to enforce labor standards in the construction industry.
The International Labor Organization (ILO) estimated that, at the end of 2006, 90% of the 40 million construction workers in China were migrant workers.
Many of the migrant workers turned to construction work after their farming communities collapsed into poverty.
In the United States, illegal immigrant labor is prevalent in the construction industry.
Because of the questionable legal status of these workers, employers often have the ability to commit crimes such as wage theft and violation of workplace standards without fear of facing consequences. 
The first unions established in the United States in the early nineteenth century tended, by nature of the industries in which their members worked, to be craft unions: shoemakers, cordwainers (shoemakers who work with cordovan leather) and typesetters all worked, as a rule, in small shops in which they had little contact with workers in other fields.
Some of these early unions also came out of a guild tradition, in which skilled workmen often owned their own shops or, if they worked for another, had a good deal of control over how the work was done, which they policed by maintaining standards for admission into the trade, requiring entrants to go through an apprenticeship program controlled by the union, rather than the employer, and dictating the processes, tools, standards and pace of work.
These traditions persisted into the twentieth century in fields such as printing, in which the International Typographical Union would enforce its own rules determining how work was done in union shops, and in the construction industry.
The concept of organizing a strong federation on the basis of craft evolved out of conflict between the Knights of Labor (KOL), which organized mass organizations of unskilled, semiskilled and skilled workers by territory, and the American Federation of Labor (AFL), which organized only skilled workers.
The craft workers were capable of demanding more from their employers due to their skills, and therefore organized into stronger organizations pursuing narrower interests.
The AFL was formed as a direct result of the perceived need by skilled workers to defend their individual craft organizations from poaching by the Knights of Labor.
The Knights of Labor believed that skilled workers should dedicate their greater leverage to benefit all workers.
Selig Perlman wrote in 1923 that this resulted in "a clash between the principle of solidarity of labor and that of trade separatism."
The trade unions "declared that their purpose was 'to protect the skilled trades of America from being reduced to beggary'.
"In 1901, the AFL issued a statement referred to as the Scranton Declaration, which asserted that unions were formed on the basis of the trade practiced by a group of skilled workers.
The Scranton Declaration would be invoked – except in the case of powerful industrial unions that resisted, such as the United Mine Workers – to enforce craft autonomy as the cornerstone of the organization.
The principle of craft autonomy began to give way in many trades, however, with the advent of industrialization in the second quarter of the twentieth century.
The most impressive example was in the textile industry, which created massive new factories staffed by unskilled workers that displaced the small scale and home workshops of weavers in New England.
New industrial processes and markets also gave rise, however, to many small shops in which semiskilled and unskilled workers did a discrete portion of the work that a skilled worker might have done a decade earlier.
The wholly new industry of ready-made clothes, as an example, replaced the workshops run by established master tailors with small operations where unskilled workers were "sweated" – a term that entered popular usage in the middle of the nineteenth century – to produce clothes for all classes of customers, from slaves to gentlemen.
Gender and ethnicity also played a part in these new patterns of work: the cotton and woolen mills in New England hired primarily young unmarried girls, often straight from the farm, to tend their machines, while sweatshops most frequently exploited immigrant workers.
Those workers who could hold on to their skill and their control over work processes, such as carpenters, butchers and printers, resisted by forming craft unions.
They not only extolled the dignity of work and the dignity of the master worker, but frequently defined themselves by what they were not: to that end, craft unions often developed rigid entrance requirements that excluded women, immigrants, African-Americans and other minority workers.