Bear in mind that the upper bound 5% is the absolute maximum, under the most generous assumptions possible, and includes every injury including minor scrapes and bruises.

It's based on USA data, so the helmet use of particular localities don't factor into it. However, if you could check the accident rate in your locality with 80% helmet use - injury accidents on the street per total cyclists in a year - it would serve as a reality check. I strongly suspect that it would still be close to 1%, just like the overall data.

If you have a 1%, or 5%, as the percentage of cyclists who have an injury accident in a year, now look for the percentage of the injuries that involve a head injury. Where a helmet might help. It's not all of them, not even close. We do have to go with Hospital Emergency numbers for this, no choice. It won't give us exact numbers because of the well known under-reporting problem, but it does provide a decent look at the proportions.

This is a table "Ten most common injuries in bicycle injury related ED visits, 2009" from a statistical brief from the National Center for Biotechnology Information .

I'll post the table here:
[TABLE="class: no_bottom_margin"]
[TR]
[TH="bgcolor: #F0F0F0, align: center"]Principal diagnosis[/TH]
[TH="bgcolor: #F0F0F0, colspan: 3, align: center"]Bicycle-related ED visits[/TH]
[/TR]
[TR]
[TH="bgcolor: #F0F0F0, align: center"]Rank[/TH]
[TH="bgcolor: #F0F0F0, align: center"]Number[/TH]
[TH="bgcolor: #F0F0F0, align: center"]Percentage[/TH]
[/TR]
[TR]
[TD]All bicycle injury related visits[/TD]
[TD="align: center"]--[/TD]
[TD="align: center"]418,700[/TD]
[TD="align: right"]100%[/TD]
[/TR]
[TR]
[TD]Superficial injury; contusion[/TD]
[TD="align: center"]1[/TD]
[TD="align: center"]108,200[/TD]
[TD="align: right"]26%[/TD]
[/TR]
[TR]
[TD]Fracture of upper limb[/TD]
[TD="align: center"]2[/TD]
[TD="align: center"]66,500[/TD]
[TD="align: right"]16%[/TD]
[/TR]
[TR]
[TD]Open wounds of head; neck; and trunk[/TD]
[TD="align: center"]3[/TD]
[TD="align: center"]50,600[/TD]
[TD="align: right"]12%[/TD]
[/TR]
[TR]
[TD]Sprains and strains[/TD]
[TD="align: center"]4[/TD]
[TD="align: center"]40,800[/TD]
[TD="align: right"]10%[/TD]
[/TR]
[TR]
[TD]Open wounds of extremities[/TD]
[TD="align: center"]5[/TD]
[TD="align: center"]28,400[/TD]
[TD="align: right"]7%[/TD]
[/TR]
[TR]
[TD]Intracranial injury[/TD]
[TD="align: center"]6[/TD]
[TD="align: center"]21,300[/TD]
[TD="align: right"]5%[/TD]
[/TR]
[TR]
[TD]Fracture of lower limb[/TD]
[TD="align: center"]7[/TD]
[TD="align: center"]14,400[/TD]
[TD="align: right"]3%[/TD]
[/TR]
[TR]
[TD]Joint disorders and dislocations; trauma-related[/TD]
[TD="align: center"]8[/TD]
[TD="align: center"]9,500[/TD]
[TD="align: right"]2%[/TD]
[/TR]
[TR]
[TD]Skull and face fractures[/TD]
[TD="align: center"]9[/TD]
[TD="align: center"]6,600[/TD]
[TD="align: right"]2%[/TD]
[/TR]
[TR]
[TD]Crushing injury or internal injury[/TD]
[TD="align: center"]10[/TD]
[TD="align: center"]3,800[/TD]
[TD="align: right"]1%[/TD]
[/TR]
[/TABLE]


At most, 19% of the injuries are what you're concerned about. That's considering ALL of the "Open wounds of head, neck, and trunk" to be only "head" and of course they're not.

So this maximum of 19% head related injuries, times your maximum of 5% chance of injury, and you're back to under 1% again. Again, these are upper bound maximums with crazy generous assumptions just to see how high the number can get, and it's still under 1% chance of a head injury in a year. A more reasonable estimation is surely much less than this.

That's still not the end of it. The under 1% is only the chance of having a (head-related) injury accident. For a picture of how much a helmet helps those odds we have to multiply this <1% by some percentage representing helmet effectiveness in head injuries. That's where it gets murky (and, again, why I wouldn't let the other guy just combine "effectiveness" with the probability of an accident). The oft-quoted 85% figure has of course been discredited. I'm not going to cite sources on this part, because I hope to avoid the straw-man arguments that always pop up on both sides when this comes up, but there is a range of credible estimates on what percentage of injuries are mitigated or prevented by helmets. The broadest range of even remotely credible estimates are approximately from 40% to 65%.

So the helmet risk mitigation is multiplying 1% by 40% to 65%, or the maximum is under 0.65%, which is the chances that a helmet will help a member of "all cyclists" during a year of riding. With every assumption made in order to maximize this risk number, unreasonably generous to the risk side. Is that still over the top risk for you?