delivered the opinion of the Court.
Wе granted certiorari to review the decision of the Colorado Court of Appeals in
Union Carbide Corp.
v.
Industrial Commission,
Roy Benally worked as a uranium miner from 1955 to 1970 and died of lung cancer resulting from exposure to radioactive materials. This dispute is between his last two employers over which of them should pay the initial $7,500 of occupational disease benefits awarded to his dependents.
Petitioner Union Carbide Corporation hired Benally on April 28, 1970, subject to the provision that he pass the pre-employment physical examination. Benally actually worked eight days before the results of the рhysical examination were known. Because the sputum test showed the onset of lung cancer, Benally was dismissed on May 5. During his eight days of employment, Benally worked forty-eight hours, twenty-six of those underground, and was exposed to 0.15 Working Level Months (WLMs) of radiation. This exposure was estimated to be less than one ten-thousandth of his total exposure to radiation during his mining career.
The referee held that Benally was “last injuriously exposed” to radioactive materials during the eight-day period that he worked for Union Carbide. Benally had previously worked for the respondent Climax Uranium Corporаtion for the four years before he went to work for Union Carbide. However, under the “last injurious exposure” rule in section 8-60-113(1), C.R.S. 1973, 1 Union Carbide and its insurer were found to be solely liable for the $7,500 in benefits. The Industrial Commission adopted this opinion and the court of appeals affirmed.
*59 I.
The issue presentеd to this court is whether Roy Benally’s eight-day exposure to radioactive materials at the Union Carbide mine is a “last injurious exposure” sufficient to hold Union Carbide and its insurer liable for the initial $7,500 in occupational disease benefits.
The pertinent section of the Occupational Disease Disablility Act provides that the employer in whose employment the employee was “last injuriously exposed” to the hazards of such disease is liable with its insurance carrier for occupational disease benefits. Section 8-60-113(1), C.R.S. 1973. 2 The Act defines “injurious exposure” as “* * * that concеntration of toxic material which would, independently of any other cause including the previous physical condition of the claimant, produce or cause the disease for which claim is made.” Section 8-60-102(8), C.R.S. 1973. 3 The Act specifies that the employee must have worked for the “last” еmployer at least sixty days in order to hold that employer liable for disability from silicosis, asbestosis, or anthracosis. However, no minimum employment time is designated for diseases caused by exposure to radioactive materials.
In the case of
Climax Uranium Co. v. Smith,
We approve the Smith test because it focuses on the harmful nature of the concentration of the radioactive exposure if continued indefinitely at that level. This interpretation is consistent with the statutory definition of “last injurious exposure,” which renders immaterial to the finding of liability the length of the period of employment with that particular employer. 4
*60 The petitioner correctly points out that many occupational diseases, such as lung cancer caused by exposure to radiation, result from a cumulative expоsure to harmful concentrations over a working career with many employers. At first glance, it seems to be contradictory to recognize that the actual length of exposure is crucial in contracting an occupational disease, but then to hold that it is immaterial in determining liability for the disease. Nonetheless, on closer analysis, this rule makes good sense.
In order to receive any occupational disease benefits, the employee must prove that he has contracted an occupational disease and that it was caused by his employmеnt activities. If the employee has worked for many different employers, it may well be that no single exposure with any one employer was in fact sufficient, in itself, to cause the disease, even though all the exposures contributed to the final result. Under such circumstances, a test for liability bаsed upon the employee’s actual length of exposure with each employer could well deny the employee any recovery for the disease. In contrast, the “last injurious exposure” rule looks at the concentration of radiation received during the last emplоyment to determine whether the employee was exposed to a harmful quantity.
In addition, the general assembly, in enacting the “last injurious exposure” rule, was certainly aware of the practical impossibility of requiring a disabled employee to ascertain the exact amount that each employer contributed in causing his disease. Thus, the employee is not required to pinpoint exactly which employer
most
injuriously exposed him to radiation, but rather he is allowed to recover from the “last” employer which injuriously exposed him. If the rule were otherwise, the employee would be burdened with the almost impossible task of apportioning liability among his several employers.
See Colorado Fuel & Iron Corp.
v.
Alitto,
II.
In the present case, Union Carbide contends that Roy Benally did not receive a “last injurious exposure” in its mine. It argues that the eight-day exposure of 0.15 Working Level Months (WLMs) of radiation — approximately one ten-thousandth of his total exposure to radiation while mining — was not harmful. Additionally, Union Carbide points out that this amount of radiation, when projected over a year, was within the 12 Working Level Months federal standard in effect at the time.
The foregoing analysis of the “last injuriоus exposure” rule has made clear that the length of the actual exposure, in this case eight days, is immaterial to the imposition of liability. Nonetheless, it must be determined *61 if a concentration of 0.15 WLMs of radiation during an eight-day period is an “injurious exposure.” If it is not an injurious exposure, then Union Carbide cannot be held liable for occupational disease benefits.
A WLM is the equivalent of one person working eight hours a day for the normal number of work days in a month, and being continuously exposed during that work period at an average of “one work level” of radiation. 5 The federal government has adopted a standard which attempts to set the maximum radiation dosage in WLMs that a miner could safely receive in a year’s time without running the risk of developing an occupational disease such as lung cancer. A standard of 12 WLMs was adopted in 1967, but was revised downward to 4 WLMs on July 1,1971. At the time that the refеree rendered his decision in this case, the federal standard was 4 WLMs.
The trier of fact must determine whether a certain concentration of radiation constitutes an “injurious exposure.” The test from the Smith case, which we adopt, is whether the amount of radiation is “a concentration of toxic material which would be sufficient to cause the disease in the event of prolonged exposure to such concentration.” 6 The federal standard, which attempts to set a level beyond which an exposure becomes injurious, is perhaps of the greatest value to the trier of fact. In addition, expert medical testimony may be helpful in deciding whether a given exposure was “injurious” to a certain person.
In the present case, the referee’s decision that the exposure was “injurious” has ample support in the record. Although there is some confusion in the record, petitioners concede in their brief that, when projected over the period of one year, Benally’s exposure to radiation at Union Carbide’s mine was equivalent to at least 6.8 WLMs. 7 This concentration is seventy percent more than the exposure lеvel permitted by the federal standard in effect at the time that the referee rendered his decision. 8
*62 In addition, the medical and epidemiological expert at trial gave the following testimony:
“Q. Would the exposure of .15 working-level months, accrued within a period of 24 hours of mining underground, аnd continued at that level for an indefinite period of time, cause a lung cancer?
“A. To answer this question directly, I would say yes. If this level of exposure were continued indefinitely, say for 20 or 30 years, that the man’s risk of lung cancer would be increased.”
This evidence clearly demonstrates thаt Benally received a “last injurious exposure” to radiation at the Union Carbide mine.
We have often recognized that: “[T]he Workmen’s Compensation Act is to be liberally construed to accomplish its humanitarian purpose of assisting injured workers and their families.”
Garner
v.
Vanadium Corp. of America,
III.
Union Carbide also contends that the application of the “last injurious exposure” rule to it in these circumstances violates its due process and equal protection rights. We find both contentions unpersuasive.
First, the petitioner argues that holding it liable for the $7,500 in benefits is an arbitrary and unreasonable taking of its property. We have interpreted the “last injurious exposure” rule to require the presence of a harmful exposure to toxic materiаl by that employer. A harmful exposure, even of only an eight-day duration, is not an arbitrary basis upon which to impose liability up to $7,500. Rather, it is a reasonable method for providing initial limited compensation to an employee and his family.
Second, the petitioner argues that the impositiоn of liability on the last employer, while absolving all previous employers from liability, violates equal protection. Since this difference in treatment is not based upon suspect classifications or fundamental interests, the issue is whether the difference in treatment bears a rational rеlationship to legitimate state objectives.
Mosgrove
v.
Town of Federal Heights,
The judgment is affirmed.
MR. JUSTICE HODGES and MR. JUSTICE GROVES do not participate.
Notes
Now section 8-51-112(1), C.R.S. 1973 (1976 Supp.).
The Colorado Occupational Disease Disability Act was repealed effective September 1, 1975, by Colo. Sess. Laws 1975, ch. 71, sec. 62 at 311. Nonetheless, this section was left substantially intact аnd is now found as part of section 8-51-112, C.R.S. 1973 (1976 Supp.). The crucial “last injurious exposure” language has not been altered.
This section was repealed with the rest of the Act and has not been reenacted.
We disapprove the language in the court of appeals’ opinion in the present case, referring to a concentration of toxic material which would “increase the probability” of lung cancer. The test in Smith, which is based on a concentration of toxic material “sufficient to cause” lung cancer, is a more appropriate interpretаtion of the statutory language.
“One work level” is the standard unit measurement of radiation and is defined by one epidemiologist as “1.3 times 10 to the 5th of potential alpha energy per liter of air.”
There has been some dispute over the “sufficient to cause” wording of the
Smith
test. The problem is thаt the legal definition is at variance with the medical definition. The medical experts do not speak of concentrations which are “sufficient to cause” occupational diseases, but rather refer to concentrations which “increase the risk” of contracting a disease. Insignificant concentrations of radiation do not “increase the risk”; in this case, the epidemiologist testified that, in his opinion, only concentrations in excess of 4 WLMs were sufficient to increase the risk of contracting lung cancer. We do not find these differences in wording to be meаningful in this area of the law. We perceive no reason to get involved in “futile searches for unattainable factual certainties.”
Mathis
v.
State Accident Insurance Fund,
The expert witness at the hearing was of the opinion that Benally received approximately 10.6 WLMs of radiation when projected over a year. The parties subsequently adopted a figure of 9.6 WLMs in framing their questions to witnesses at the hearing.
The pertinent federal standard is the standard in effect at the time of the referee’s decision, not the standard in effect at the time that the employee received the exposure. The crucial issue is whether the exposure was “injurious” to the miner, not whether the mine was being operated within the then-applicable federal standards. The most up-to-date federal standard is undoubtedly of the greatest help to the referee in deciding whether an exposure was injurious.