The critical point to be made about HPPD is to dispel the myth that is is rare. The only study ever having been done quantifying the frequency of HPPD among drug users was drafted and conducted by Dr. Matthew Baggot; PhD, Berkeley. According to the survey study, about 4% of recreational drug users experience HPPD-like symptoms severe enough to warrant medical attention and yet a staggering 60% of all participants reported persistent visual artifacts long after their trip was over.(Baggott MJ, et al. Abnormal visual experiences in individuals with histories of hallucinogen use: a web-based questionnaire. Drug Alcohol Depend. 2011 Mar 1; 114(1):61-67). This alludes to the notion that the classification of HPPD into two distinct groups (Type 1 which is benign and Type 2 which is severe) holds true and also explains why it is grossly under reported. Those who belong to the Type 1 subgroup rarely complain. Some even consider  HPPD a gift and are not at all pressed to seek medical attention whereas Type 2 patients who suffer horribly from HPPD, despite seeking medical attention, risk being misdiagnosed due to the medical community being uninformed of this neuro-visual disorder, given medication that will exacerbate symptoms, and are generally left abandoned and misunderstood. Which has unfortunately lead those who feel that they have no recourse to suicide. Make no mistake, HPPD is NOT a rare disorder. On the contrary, it is a condition that physicians either aren’t aware of or aren’t trained to treat. Underreporting of individuals inflicted with HPPD has become more and more of a pressing issue because individuals within the Type 1 population go through life unhindered by the lasting visual artifacts from a trip, which are not as bothersome as Type 2 patients, and believe this state is normal. It is not. HPPD is not normal, and it is not rare. 

According to the 2013 National Household Survey on Drug Abuse (NHSDA), an estimated 1.3 million persons aged 12 or older (0.5 percent) used hallucinogenic drugs within the past month. Among 18 to 25 year olds in 2013, the rate of current use of hallucinogens was 1.8 percent. First-time users aged 12 or older numbered 1.1 million persons. The incidence of first-time hallucinogen use has exhibited two prominent peaks of increase: 1) From 1965-1969 there was a marked increase predominately in the use of LSD, and 2) from 1992-2000 there was a marked increase predominately from use of MDMA. However, hallucinogens continue to be among the most frequently abused class of drugs in high school students, after alcohol and marijuana.

In taking that percentage and running the numbers of drug users as seen in this 2013 study (the only one of its kind provided by the NIH):

http://www.drugabuse.gov/publications/research-reports/hallucinogens-dissociative-drugs/why-do-people-take-hallucinogens

The study purports that hundreds of thousands of children under 12 years old nationally have experimented with hallucinogenic drugs on a monthly basis which statistically makes it impossible for HPPD to be uncommon; in fact there are probably thousands upon thousands of cases that are misdiagnosed as most physicians do not understand what HPPD is, let alone how to treat it.

Race, sex, and age-related demographics

Within the United States, hallucinogen use is most common amongst non-Hispanic whites. There is also a high life-time prevalence rate of hallucinogen use among American Indian and Alaska Natives. Hallucinogens are abused most frequently by males. The highest rate of hallucinogen abuse occurs in persons aged 18-25 years.