Contact dermatitis from paraphenylenediamine (PPD) in hair dye causes patchy alopecia in approximately 0.5% of dye users. Allergic reactions to Minoxidil, shampoos, conditioners, and styling products can also trigger inflammation-driven hair loss. Tracking density before, during, and after a contact dermatitis episode creates the clinical timeline your dermatologist needs to distinguish allergic alopecia from androgenetic alopecia, telogen effluvium, or other causes.
How Contact Dermatitis Causes Hair Loss
Contact dermatitis comes in two forms, and both can affect hair density.
Allergic contact dermatitis (ACD) is an immune-mediated reaction. When your skin encounters an allergen it has been sensitized to, the immune system launches an inflammatory response. On the scalp, this inflammation damages follicles and pushes hairs into the telogen (resting/shedding) phase prematurely. Hair loss typically appears 2 to 4 weeks after the allergic exposure.
Irritant contact dermatitis (ICD) results from direct chemical damage to the skin barrier, not an immune response. Strong chemicals like bleach, high-concentration peroxide, or improperly formulated topical treatments can cause ICD. The resulting damage may thin hair in the affected area.
The key difference for tracking: ACD tends to produce patchy hair loss in the specific area of allergen contact, while ICD produces more diffuse thinning with visible scalp damage.
Common Allergens That Cause Scalp Dermatitis and Hair Loss
| Allergen | Found In | Reaction Rate |
|---|---|---|
| Paraphenylenediamine (PPD) | Hair dye | ~0.5% of users |
| Propylene glycol | Minoxidil solutions, conditioners | 1 to 3% of users |
| Methylisothiazolinone (MI) | Shampoos, conditioners | Rising sensitization rate |
| Fragrances | Most hair products | Most common contact allergen overall |
| Cocamidopropyl betaine | Shampoos, cleansers | Increasing reports |
| Nickel | Hair clips, bobby pins | ~10% of women |
Minoxidil allergy deserves special attention. Propylene glycol, a common carrier in liquid Minoxidil solutions, causes contact dermatitis in 1 to 3% of users. This is particularly problematic because the resulting inflammation and shedding can be confused with the normal initial Minoxidil shed that occurs in the first 2 to 8 weeks of treatment.
How to Track Contact Dermatitis Hair Loss
Step 1: Document the Allergen Exposure Event
As soon as you suspect a contact dermatitis reaction, record:
- Date of allergen exposure
- Product used and its ingredient list
- Affected scalp areas (photograph them)
- Symptoms: redness, itching, swelling, burning, blistering
- Severity on a 1 to 10 scale
- Any treatment applied (topical steroids, antihistamines)
Step 2: Capture Baseline Density of Affected Areas
Use myhairline.ai to photograph the affected areas immediately. Hair loss from contact dermatitis does not appear instantly. It typically takes 2 to 4 weeks for the inflammatory damage to push follicles into telogen shedding. Your baseline photos capture density before the shedding begins.
Step 3: Track the Shedding Phase (Weeks 2 to 8)
During the shedding phase, take density photos every week. Log:
- Daily hair count in shower drain or on pillowcase
- Whether shedding is localized to the contact area or spreading
- Ongoing scalp symptoms (inflammation duration and severity)
- Any treatments prescribed by your dermatologist
- Whether you have eliminated the offending allergen
Step 4: Track the Recovery Phase (Months 2 to 6)
Once the allergen is eliminated and inflammation resolves, follicles typically begin recovering. Continue biweekly density photos and log:
- First signs of new hair growth (vellus hairs in affected area)
- Density measurements compared to pre-episode baseline
- Any scarring or permanent follicle damage (flat, shiny patches with no regrowth)
Distinguishing Allergic Alopecia from Other Causes
Your tracking data helps separate contact dermatitis hair loss from other conditions:
| Feature | Contact Dermatitis Alopecia | Androgenetic Alopecia | Telogen Effluvium |
|---|---|---|---|
| Pattern | Localized to contact area | Diffuse, patterned (Norwood) | Diffuse, all over |
| Onset | 2 to 4 weeks after exposure | Gradual over months/years | 2 to 4 months after trigger |
| Scalp symptoms | Redness, itching, swelling | None typically | None typically |
| Recovery | Full recovery once allergen removed | Progressive without treatment | Self-resolving in 6 to 12 months |
| Inflammation visible | Yes | No | No |
If your tracking data shows localized hair loss with concurrent scalp inflammation in the area where a product was applied, contact dermatitis is the likely cause.
Minoxidil Allergy: A Special Case
If you are tracking Minoxidil treatment (40-60% efficacy for regrowth) and notice scalp inflammation alongside shedding, you need to determine whether the shedding is:
- Normal Minoxidil-induced shedding: Occurs in weeks 2 to 8, involves increased hair fall without scalp irritation, and resolves on its own
- Allergic contact dermatitis from Minoxidil: Involves redness, itching, burning, and flaking in the application area, does not resolve, and may worsen with continued use
Your tracking data should log both density measurements and scalp symptom scores. If density is declining and inflammation is present, stop Minoxidil and consult your dermatologist. Switching to a foam formulation (which lacks propylene glycol) often resolves the allergy while maintaining treatment benefits.
Recovery Timeline: What Your Data Should Show
Most contact dermatitis alopecia recovers fully once the allergen is eliminated:
- Weeks 1 to 4 after allergen removal: Inflammation resolves, shedding slows
- Months 1 to 3: New vellus (fine) hairs appear in affected areas
- Months 3 to 6: Vellus hairs mature into terminal hairs, density approaches baseline
- Month 6+: Full recovery in most cases, unless scarring occurred
If your tracking data shows no recovery after 6 months of allergen avoidance, the dermatitis may have caused scarring alopecia, which requires different treatment.
Medical Disclaimer
This article is for informational purposes only and does not constitute medical advice. Contact dermatitis can cause significant scalp damage if untreated. If you suspect an allergic reaction on your scalp, stop using the suspected product immediately and consult a board-certified dermatologist. Patch testing can identify your specific allergens.
Document Your Scalp Health Over Time
Start tracking your scalp condition at myhairline.ai/analyze. Whether you are monitoring a current dermatitis episode or establishing a healthy baseline, consistent density data gives you and your dermatologist the evidence needed to make informed treatment decisions.