In this workmen’s compensation case appellee, Deputy Commissioner, concluded that the collapse and death of appellant’s late husband, Edward E. Wheatley, on February 12, 1964, “did not arise out of and in the course of the employment.” Her action to set aside this decision1 was dismissed by the District Court, which granted summary judgment to appellee. We reverse.
I
The testimony established that the employee began his last day like any other work day. At 8:48 a. m. he punched in at Associated Transport Company, the trucking concern where he had been employed as a mechanic for seventeen years. He changed into work clothes and assembled his tools before starting in on his first repair job. Apparently before beginning actual repair work, Wheatley walked out into the yard adjacent to the garage to answer a call of nature. The “bathroom is way in the back of the terminal” and Wheatley, crippled in one leg, never “done much about climbing the steps,” and “didn’t do too much running around.” It was an average day in winter, temperature about 40 degrees.
At about 9:30, Wheatley was observed heading back to the garage. About 40 feet from the shop, and without any visible reason, he collapsed. Aid was fruitless. The death certificate recited that death occurred at the hospital at 10:22 a. m., and this time was accepted by appellee. The autopsy report stated that Wheatley was suffering from marked arteriosclerotic heart disease, and appellee found that death resulted from myocardial insufficiency.
As appellee found, prior to the collapse neither the employee’s wife nor his supervisor nor any co-worker had heard any suggestion from him that he was suffering from an ailment, nor did his appearance that morning suggest to any of them that he was suffering from any ailment or difficulty. His widow’s uncontradicted testimony establishes that he had never had a heart problem that he was aware of prior to the date of his death, nor any ailments other than lumbago and an occasional cold. A week prior to his death his face was pink when he came home, and when she asked him about it he said it was cold weather in the mountains where he had been working on a trailer truck.
Appellee’s order included these statements as his crucial findings of fact: On the morning of February 12, the employee “was not subject to any employment-related emotional disturbance or to any significant physical exertion.” His collapse and death were caused by a myocardial insufficiency, due to a preexisting advanced arteriosclerotic heart disease, which was “neither caused nor aggravated by the employment on February 12, 1964 or prior thereto.” The myocardial insufficiency “resulted from the natural progression of the arteriosclerotic heart disease.”
Although judicial review of workmen’s compensation proceedings is *310limited,2 the courts must set aside compensation orders, including denials of claims, when “not in accordance with law.” 33 U.S.C. § 921(b) (1964). An administrative order must be set aside if it rests on factual premises not based on substantial record evidence or if the agency’s underlying standards are not in accord with law. With this in mind we examine the record and premises underlying appellee’s order.
Claimant’s medical expert, Dr. Chapman, testified without contradiction, on examination of the coroner’s autopsy report, and of the microscopic slides obtained from the morgue (which revealed an absence of a significant degeneration of heart tissue), that the attack could have begun no more than one and one-half hours before death, and probably began less than an hour before then. The seizure therefore began after Wheatley arrived at work.3 Claimant’s expert further testified that such an attack is always preceded by some precipitating event, and that the strain of urinating on a cold day could have brought it on.
Employer’s counsel called Dr. Thomas, and assuming in hypothetical form the salient events of the morning (including the urinating in the cold), asked whether he had an opinion “as to whether or not the death arose out of or in the course of the employment.” Dr. Thomas testified that, on the basis of the assumptions stated and the autopsy report, he did have such an opinion. (He did not refer to, and it was later brought out, J.A. 19, that he had not examined, the coroner’s microscopic slides that Dr. Chapman found material.) In his opinion the death “was not the result of any activity involved in this man’s employment.” He was then asked to state his reasons, and responded (J.A. Ill):
Well, according to the autopsy report this man had generalized arteriosclerosis with significant narrowing of the coronary arteries, and, as Dr. Chapman has indicated to you, anything can cause a sudden demise under these circumstances.
And, as he has pointed out, this can happen in the course of a patient sitting in bed or lying in bed or relaxing or sitting in a chair watching television, without necessarily having any exertion.
The absence of any specific stimulating or exertional episode makes me feel that the attack was in no way related to his employment.
It is conceivable that the mere fact of urinating and the stress and strain of trying to urinate on a cold day —which also is a factor, because coronary artery disease is affected adversely by cold, it produces constriction of blood vessels and it may increase the tendency or the propensity towards heart attacks or sudden death — these factors alone could have been sufficient to produce this death rather than any activity that the individual was in at the time of his death.
Employer’s counsel went on to ask whether it was requisite that there be stress or trauma as a factor precipitating heart disease. Dr. Thomas replied that it was not, and added (J.A. 112/3):
On the information, or from the information that I was allowed to read, I would say that there was nothing in the history or the events that were given to me that would make me feel that anything that happened to the deceased on that particular morning may have been a precipitating factor in that respect.
Asked on cross-examination whether he could express an opinion with reasonable medical certainty whether decedent’s activities on the job, such as the urinat*311ing, were a precipitating factor that caused death, he said that the urination could have caused a stress or strain that precipitated death. Asked whether this was more likely than not the cause, he said he could not really give a yes or no answer. Responding to a question of the commissioner, the witness said this urinating in the cold “could have been a more significant precipitating factor” than anything else the deceased did that he knew about. He was not able to mention any other more probable precipitating factor.
II
The decision and order cannot be supported by reference to the opinion of the expert called by the employer that the employee’s death “was not the result of any activity involved in this man’s employment.” His testimony is intelligible only on the assumption that he' regarded Wheatley’s urination-in-the-cold, in his own view as likely a precipitating factor as any, as not being “activity involved in this man’s employment.” The premise underlying his opinion is at core a proposition of law, not science, that lies outside the province of his expertise, and is not sustainable on this record.
The general rule of law is that accidents and deaths “occurring while an employee is on his way to or from toilet facilities, or while he is engaged in relieving himself, arise within the course of employment, subject only to the possible question * * * of the reasonableness of the means or place chosen.”4 Recovery is denied on this ground when abnormal means are chosen despite the availability of satisfactory conventional facilities, and when the means chosen are both dangerous and unreasonable in the circumstances.5 Working in this garage Wheatley used the nearby yard rather than walk, with his “stiff” leg, up a number of steps and across the building to the bathroom in the back. There was no hint that Wheatley had not made customary resort to these ad hoc facilities or that he had been forbidden to do so. Wheatley’s use of the yard was not only for his convenience, but may have benefited his employer through the time saved. In any event, the employer did not show that he had forbidden Wheatley’s use of the informal facilities, or that there had been no prior use of these facilities which might have put the employer, through its supervisors, on notice of the conduct and prompted a warning if such conduct was to be forbidden. On either of these possibilities, the burden was on the employer to illuminate the facts.
It would not defeat compensation even if the act of urinating, in cold weather out of doors, is taken as not involving an unusual strain. The law in this jurisdiction does not require any unusual stress, and contemplates awards so long as the death or injury results from activity in the course of employment.6 It makes no difference that the decedent was exposed to no more than the ordinary hazards of living and working, or that the same kind of injury might have occurred wherever he might have been.7
*312Taking the commissioner’s decision as we find it,8 it cannot be sustained by-hypothesizing that he might have ruled that an injury or seizure which occurs while an employee is urinating in the yard outside is by that fact excluded from the coverage of the law. We need not consider whether such a standard might be permissible, notwithstanding contrary rulings already noted. If such a standard is to be the basis of decision, it must be presented with a reasoned analysis and in such form as to show that it was invoked as a standard of general application, and was not an ad hoc rule being applied obliquely for one occasion without any purpose of breadth in application.
Ill
There is an express statutory presumption that the “claim comes within the provisions of this chapter,” a presumption of compensability grounded in the “humanitarian nature” of the Act.9 Moreover, and this is significant in the present case, the fact that an injury or death occurs “in the course of employment strengthens the presumption that it arises out of the employment, with doubts resolved in the claimant’s favor.” Vendemia v. Cristaldi, 95 U.S.App.D.C. 230, 232, 221 F.2d 103, 105 (1955). The rejection of the claim cannot be supported as a matter of law unless the record contains “substantial evidence” showing that death did not arise out of employment.10
It is undisputed that the employee had a pre-existing condition of arteriosclerosis. But it is settled that an aggravation of a pre-existing condition may constitute a compensable accidental injury under the Act, Howell v. Einbinder, 121 U.S.App.D.C. 312, 313, 350 F.2d 442, 443 (1965).11 Thus the statutory presumption brings within the Act a death that results in the course of employment when a pre-existing internal disorder takes a sudden turn for the worse, unless the record contains substantial evidence as to the cause of the collapse which shows that it was not aggravated or precipitated by a work-related factor. Butler v. District Parking Management Co., 124 U.S.App.D.C. 195, 363 F.2d 682 (1966). Complaints have been voiced against the aggravation rule as applied to cardiac cases, and suggestions made for narrowing compensability.12 But these would require changes in our long-standing construction of the Act, and they are appropriately addressed to the Congress.
The record plainly contains testimony that is in accordance with the thrust of the presumption, for sustaining the claim. The question is whether there is substantial evidence in the record to dispel the presumption. After careful analysis we conclude that neither the testimony of Dr. Thomas, relied on by appellee, nor anything else in the record constitutes such substantial evidence, and that hence the order is without adequate record support.
Dr. Thomas testified on the basis of the autopsy report, which stated that Wheatley had “marked coronary arteriosclerosis.” He agreed that all persons aged 60 have arteriosclerosis “in varying degrees.” The word “marked” “would *313connote to my mind a more severe degree” than “medium or mild,” though he did not know whether it “means one centimeter or five centimeters.” (J.A. 114).
Dr. Thomas testified: “With * * * reasonable medical certainty, I would say that the coronary arteriosclerosis was the major reason for this man’s having the attack.” (J.A. 117) This followed: “Q. But can you tell us what * * * pushed it over the brink, Doctor? A. What the precipitating event was, I cannot say.” (J.A. 117). So he was unable to state an opinion on the key question whether there was a work-related factor that aggravated the employee’s condition.
He testified that it was possible that death occurred even in the absence of stress or strain. He also testified that there is a greater possibility of sudden death in the absence of stress for a patient who has narrowing of coronary blood vessels as indicated in the autopsy report than for a patient who doesn’t have any coronary heart disease (J.A. 112). But he did not testify in any way as to the probability, even for a person with coronary disease, of death in the absence of stress or strain, whether the probability was 90-10, or 10-90, or 50-50.
As appears from this expert’s testimony quoted above (from J.A. Ill), his own explanation for his conclusion that death was not the result of any employment activity referred to urinating-in-the-cold as sufficient to produce death. His assumption that the urinating-in-the-cold was not work-related was on a point of law on which his testimony has no weight. It is significant, however, that he responded to the question of employer’s counsel asking him to explain the basis for his conclusion by volunteering a comment on the significance of urinating-in-the-cold as a meaningful medical consideration — including the effect of the cold in constricting the blood vessels and increasing propensity toward heart attacks. Obviously that medical testimony was favorable toward allowance rather than rejection of the claim.
Sometimes so-called “negative” evidence is informative. If a man has no blood in the sputum, no cough, no weakness, no headache, no elevation of temperature or pulse, no stuffiness or pain in the chest — then from all these facts, a doctor can say “with reasonable medical certainty,” or as a matter of some probability, that this man does not have pneumonia.
But here the doctor did not give an opinion as a matter of reasonable medical certainty. He did not even respond to the Commissioner’s effort to obtain an opinion in terms of probability. He was not able to say, one way or the other, either as a reasonable medical certainty or with any other estimate of probability, that the urinating-in-the-cold was not the factor bringing on the heart seizure.
The possibility that Wheatley’s death was due to urinating-in-the-cold is not a mere fancy or wisp of “might have been,” like the musing of the old judge in Whittier’s poem who was enchanted by the sight of Maud Muller. It was volunteered by Dr. Thomas as a realistic possibility providing meaningful perspective for his opinion. This certainly sufficed, in the light of the statutory presumption, to require substantial evidence in refutation, at least to articulate that this possibility was improbable — if he could say that it was improbable.
Rebutting evidence may be hard to develop, given the limits of medical ability to reconstruct why “something unexpectedly goes wrong within the human frame.” But that is precisely why the presumption was inserted by Congress. It signals and reflects a strong legislative policy favoring awards in arguable cases. We have frequently pointed out that since the statute “should be construed liberally”13 in favor of em*314ployees and their dependents, it is “in their favor [that] doubts, including the factual, are to be resolved.” 14
This approach has also characterized our application of the substantial evidence test in these cases. We have made it clear that we “will not sustain the administrative findings merely because they are substantiated by some isolated evidence. Our review must also take account of the settled rule that the Act is to be construed with a view to its beneficent purposes. Doubts, including the factual, are to be resolved in favor of the employee or his dependent family.” 15
Where there is substantial evidence on both sides of an issue the Commissioner’s finding is conclusive. But here the record does not contain substantial evidence to dispel the statutory presumption.
Reversed.
. The action was brought under the provisions of the Longshoremen’s and Harbor Workers’ Compensation Act of March 4, 1927, 44 Stat. 1424, as amended, 33 U.S.C. §§ 901-950, made applicable to the District of Columbia by D.C. Code § 36-501.
. Cardillo v. Liberty Mut. Ins. Co., 330 U.S. 469, 477, 67 S.Ct. 801, 91 L.Ed. 1028 (1947); Hurley v. Lowe, 83 U.S. App.D.C. 123, 125-126, 168 F.2d 553, 555-556, cert. denied, 334 U.S. 828, 68 S.Ct. 1338, 92 L.Ed. 1756 (1948).
. This is so even if the time of death is taken at 9:45 a. m. as stated in the autopsy certificate, and a fortiori if it was 10:22 a. m., as the Commissioner found.
. 1 A. Larson, Workmens Compensation Law § 21.53 at 321 (1965).
. See id. at 21.82-21.84 at 340-48.
. See Commercial Cas. Ins. Co. v. Hoage, 64 App.D.C. 158, 159, 75 F.2d 677, 678. cert. denied, 295 U.S. 733, 55 S.Ct. 645. 79 L.Ed. 1682 (1935):
It has been held a number of times, and we think correctly, that an accidental injury may occur notwithstanding the injured is then engaged in his usual and ordinary work, and likewise that the injury need not be external. It is enough if something unexpectedly goes wrong within the human frame.
. See Hartford Accident & Indem. Co. v. Cardillo, 72 App.D.C. 52, 54, 112 F.2d 11, 13, cert. denied 310 U.S. 649, 60 S.Ct. 1100, 84 L.Ed. 1415 (1940); New Amsterdam Cas. Co. v. Hoage, 61 App.D.C. 306, 307, 62 F.2d 468, 469, cert. denied, 288 U.S. 608, 53 S.Ct. 400, 77 L.Ed. 982 (1933).
. SEO v. Chenery Corp., 318 U.S. 80, 63 S.Ct. 454, 87 L.Ed. 626 (1943).
. 33 U.S.C. § 920(a) (1964); O’Keeffe v. Smith, Hinchman & Grylls Assoc., 380 U.S. 359, 362, 85 S.Ct. 1012, 13 L.Ed. 2d 895 (1965).
. Del Vecchio v. Bowers, 296 U.S. 280, 56 S.Ct. 190, 80 L.Ed. 229 (1935) ; Butler v. District Parking Management Co., 124 U.S.App.D.C. 195, 363 F.2d 682 (1966).
. “Thus the cases almost invariably decide that the fact that the injury would not have resulted but for the pre-existing disease, or might just as well have been caused by a similar strain at home or at recreation, are both immaterial.” H. McNiece, Heart Disease and the Law 12 (1961). See also id. at 57-59.
. See generally Larson, The “Heart Cases” in Workmen’s Compensation; An Analysis and Suggested Solution, 65 Mich.L.Rev. 441 (1967); H. McNiece, supra note 11, at 110-138 (1961).
. Baltimore & Philadelphia Steamboat Co. v. Norton, 284 U.S. 408, 414, 52 S.Ct. 187, 76 L.Ed. 366 (1932) (Butler, J.). “The law’s humanitarian purpose is to ensure that all shall be compensated, regardless of the employer’s fault.” Jack*314son v. Lykes Bros. S. S. Co., 386 U.S. 731, 736, 87 S.Ct. 1419, 1423, 18 L.Ed.2d 488 (1967). This expression in Justice Stewart’s dissent was not controverted in the majority opinion.
. See J. Y. Vozzolo, Inc. v. Britton, 126 U.S.App.D.C. 259, 377 F.2d 144, 147 (1967); Howell v. Einbinder, 121 U.S.App.D.C. 312, 314, 350 F.2d 442, 444 (1965); Hancock v. Einbinder, 114 U.S.App.D.C. 67, 70, 310 F.2d 872, 875 (1962); Phoenix Assurance Co. of New York v. Britton, 110 U.S.App.D.C. 118, 120, 289 F.2d 784, 786 (1961).
. Friend v. Britton, 95 U.S.App.D.C. 139, 141, 220 F.2d 820, 822 (1955).