Petitioners, as surviving dependents of decedent, seek review of the final order of the Industrial Commission affirming the hearing officer’s denial of benefits to petitioners on their claim arising from the decedent’s death by a heart attack. We affirm.
Decedent, Don J. Kohler, was employed by respondent Neilson’s, Inc. (employer), from 1972 until his death in 1979. Employer was primarily engaged in supplying crushed rock to construction sites, which it did through three rock crushing operations in Colorado and New Mexico. For most of his career with employer, decedent was a superintendent of one of the rock crushing operations. Approximately three months prior to his death, he was promoted to general superintendent of all three crushers.
On Saturday, July 14, 1979, decedent travelled from Cortez, Colorado, to the crusher at Shiprock, New Mexico, to supervise the operation in the absence of the regular superintendent, who was ill. At Shiprock, decedent attended to various supervisory duties. Upon learning that the employees were in need of drinking water, decedent offered to drive into town to get more water. He was later found unconscious in his car, and was transported to a local hospital, where he subsequently died.
Petitioners applied for workmen’s compensation death benefits. Affirming a referee’s order, the Industrial Commission denied benefits on the basis that the exertion of claimant’s job at the time of the heart attack was not “unusual” and that therefore the heart attack was not an “accident” or “injury” within the meaning of the Workmen’s Compensation Act. The critical portion of the referee’s findings is as follows:
“The referee finds that the activities engaged in by the claimant during the week preceeding his death were the same as those he frequently encountered and experienced in his job. Therefore, the exertion of his job was not unusual, and his heart attack is not an accident or an injury within the meaning of C.R.S., 1973, 8-41-108.”
I.
On review, the petitioners first contend that the evidence before the hearing officer established that decedent’s activities prior to his death constituted “unusual exertion” within the meaning of the workmen’s compensation act, and that therefore decedent’s death is compensable. We disagree.
Section 8-41-108(2.5), C.R.S.1973 (1982 Cum.Supp.), provides as follows:
“ ‘Accident,’ ‘injury,’ and ‘occupational disease,’ shall not be construed to include disability or death caused by heart attack unless it is shown by competent evidence that such heart attack was proximately caused by unusual exertion arising out of and within the course of the employment.” (emphasis supplied)
In City & County of Denver v. Industrial Commission, 195 Colo. 431, 579 P.2d 80 (1978), the Supreme Court, construing the “unusual or extraordinary overexertion” standard then in effect, upheld an award of benefits to a police sergeant who had suffered a heart attack during a period of unusually long hours and extra work, with the following language:
“The unusual overexertion doctrine must be applied according to the emplloyee’s work history rather than work patterns of his profession in general.”
We view this language as being equally applicable under the present statutory standard of “unusual exertion.”
Thus, the decedent’s activities near the time of the heart attack must be compared to his normal work activities in order to determine if the former were unusual. Townley Hardware Co. v. Industrial Commission, 636 P.2d 1341 (Colo.App.1981).
In the instant case, the hearing officer received evidence of decedent’s work histo*1005ry in testimony from a variety of witnesses, including decedent’s wife and son, and several co-workers. Decedent’s wife testified that there was nothing unusual about the stress decedent was under during the week before his death. Several co-workers testified that decedent took his promotion “in stride,” that there was no difference in the way he handled his job before or after the promotion, and that on the day he died, decedent undertook no particularly stressful activity nor any physical labor. On the other hand, decedent’s son testified that decedent was “very agitated” and “preoccupied” during the week before his death. Another employee stated that during the same week, decedent was “more intense” and that “nothing pleased him.” Based upon this conflicting evidence, the hearing officer made the findings set out above.
If supported by substantial evidence, findings of the Commission based on conflicting evidence are binding on review. Prestige Homes, Inc. v. Legouffe, 658 P.2d 850 (Colo.1983). Substantial evidence supports the findings here.
II.
We also reject petitioners’ contention that the applicable law in Colorado requires only that the stress experienced by the decedent prior to this death be more than the stress of everyday life in order to constitute “unnusual exertion” within the meaning of the Act.
As discussed above, in applying the “unusual exertion” test, the proper focus is upon the activities and exertion of the employee at the time of his heart attack when compared to his normal work activities and the usual exertion attendant thereon. Townley Hardware Co. v. Industrial Commission, supra; see City & County of Denver v. Industrial Commission, supra. Hence, petitioners’ contention in this regard is without merit.
III.
Petitioners’ final contention is that insofar as § 8-41-108(2.5), C.R.S.1973 (1982 Cum.Supp.) requires a different or more severe standard for establishing that a heart attack is a compensable “accident” or “injury” than is required for other types of accidents or injuries, such provision denies petitioners equal protection of the law and is therefore unconstitutional. We do not agree.
As in all equal protection cases, this court must first determine the standard to be used in assessing the validity of the challenged legislation. Olson v. Public Service Co., 190 Colo. 512, 549 P.2d 780 (1976). Because the right to disability benefits is not a fundamental right, and because the class of employees disabled by heart attacks is not a suspect class, the rational basis test must be applied. See Stark v. Zimmerman, 638 P.2d 843 (Colo.App.1981); Stevenson v. Industrial Commission, 190 Colo. 234, 545 P.2d 712 (1976).
That test requires that the state need only show that the challenged provision is rationally related to a legitimate state interest. Bellendir v. Kezer, 648 P.2d 645 (Colo.1982); Fritz v. Regents of the University of Colorado, 196 Colo. 335, 586 P.2d 23 (1978). Thus, classes can be treated differently, so long as this unequal treatment is based on reasonable differences. Bushnell v. Sapp, 194 Colo. 273, 571 P.2d 1100 (1977). In applying that test, the statutory scheme may be invalidated only if no set of facts can reasonably be conceived to justify it. Millis v. Board of County Commissioners, 626 P.2d 652 (Colo.1981). Further, every statute is presumed constitutional unless proven beyond a reasonable doubt to be invalid. Turner v. Lyon, 189 Colo. 234, 539 P.2d 1241 (1975).
In applying the rational basis test, therefore, we must first identify the legitimate state purpose which this legislation purportedly furthers, and then determine whether such an objective is rationally furthered by the scheme or system created by the legislation. See Lujan v. Colorado State Board of Education, 649 P.2d 1005 (Colo.1982).
The purpose of the act is to provide monetary relief to employees who, while *1006performing services for the employer, suffer disability or death as a result of an accident or injury arising out of and in the course of their employment. Section 8-52-102, C.R.S.1973 (1982 Cum.Supp.); see Bellendir v. Kezer, supra. This is a legitimate state purpose. Edwards v. Price, 191 Colo. 46, 550 P.2d 856 (1976). In order to effectuate this purpose, the General Assembly, in enacting § 8-41-108, C.R.S.1973 (1982 Cum. Supp.), has chosen to define the terms “injury” and “accident” and to limit the applicability of those terms to heart attack cases where “unusual exertion” is shown. The General Assembly could have reasonably concluded that because of the inherent difficulty in ascribing the cause of many heart attacks to particular events or circumstances, a claimant, in order to receive benefits, should be required to show the presence of unusual exertion in the performance of his employment duties, and that such exertion is causally related to the heart attack.
While this legislative limitation is arbitrary in the sense that the General Assembly might have chosen some other standard by which to review heart attack cases, our inquiry in the instant case is limited to whether this statute, as presently constituted, furthers the purposes of the Act in a rational manner. Bellender v. Kezer, supra. We conclude that it does.
This view is consistent with that taken by the U.S. Supreme Court:
“In the area of economics and social welfare, a State does not violate the Equal Protection Clause merely because the classifications made by its laws are imperfect. If the classification has some ‘reasonable basis,’ it does not offend the Constitution simply because ‘... in practice it results in some inequality.’ ”
Dandridge v. Williams, 397 U.S. 471, 90 S.Ct. 1153, 25 L.Ed.2d 491 (1969) (quoted in Manor Vail Condominium Association v. Town of Vail, 199 Colo. 62, 604 P.2d 1168 (1980)).
Order affirmed.
KIRSHBAUM, J., concurs. TURSI, J., dissents.