People use "mildew" for the thin gray film on a shower wall and "mold" for the fuzzy black or green stuff in a basement, and treat them as two different enemies. The biology is simpler than the vocabulary. Here is the difference that actually matters for your house.
What is the difference between mold and mildew?
Mildew is, in everyday usage, a common name for certain molds that grow as a flat, powdery or downy film on damp surfaces, while "mold" is the broader category that includes thicker, fuzzy, raised growth. Both are fungi, both grow from moisture, and the EPA treats them under one umbrella: molds are a natural part of the environment that become a problem indoors when they grow on wet surfaces (EPA, A Brief Guide to Mold, Moisture and Your Home). The CDC likewise does not draw a health line between "mildew" and "mold" — its guidance is to remove any mold growing indoors regardless of what you call it (CDC, Mold: Basic Facts).
The honest version is that "mildew" describes how the growth looks more than it describes a separate organism you need to fear differently.
How can you tell them apart by sight?
What people call mildew is usually flat, stays close to the surface, and appears gray, white, or light brown — think of the film on grout, windowsills, or fabric left damp. What people call mold is more often raised and fuzzy, comes in black, green, blue, or orange, and tends to sink into or stain the material it grows on. The appearance is a rough field guide, not a diagnosis. The EPA and CDC both stop short of telling homeowners to identify the type, because the response is the same either way: clean it up and fix the moisture (EPA; CDC).
If the difference you actually care about is whether it is on the surface or growing into the structure, that is the question a professional answers, not the mildew-versus-mold label.
Does the difference change your health risk?
Not in a way you should manage by the name. The well-supported health effects of indoor fungal growth — for people who are sensitive — are respiratory and allergic: nasal stuffiness, throat irritation, coughing, wheezing, and asthma flare-ups (EPA, Mold and Health; WHO, Dampness and Mould). Those effects are tied to the presence and amount of growth and to individual sensitivity, not to whether the film fits the "mildew" picture or the "mold" picture. Someone with asthma can react to a mildew-looking film on a shower wall.
The practical risk driver is how much there is and how long it has been growing, which traces back to the moisture feeding it.
When is surface cleaning enough, and when isn't it?
Surface cleaning handles small, surface-only growth on hard, non-porous materials. The EPA's threshold for a do-it-yourself cleanup is roughly the small-patch range; larger affected areas, growth fed by contaminated water, or anything porous and saturated points toward professional help (EPA, A Brief Guide to Mold, Moisture and Your Home). The tell that "mildew" is actually a deeper problem is recurrence: if the same film keeps returning after you clean it, you have not fixed the moisture source, and the surface was never the real issue.
A flat film on bathroom grout that wipes away and stays gone is a maintenance task. A film that comes back every few weeks, or fuzzy growth spreading across drywall, is a moisture diagnosis waiting to happen. See condensation vs. a leak for how to tell which is feeding it.
What if the growth keeps coming back?
Recurring growth is the signal to stop scrubbing and start investigating. Because every fungus indoors traces to water, growth that returns means moisture is still arriving — through condensation, a hidden leak, poor ventilation, or humidity that never drops. The EPA's core instruction is to control moisture to control mold (EPA, A Brief Guide to Mold, Moisture and Your Home).
When the source is not obvious, a qualified mold inspector can find it. They use moisture meters and, where it is warranted, sampling to locate hidden growth and document the cause — the kind of assessment a homeowner cannot replicate with a spray bottle. See when to test for mold for when that step is worth it.
Sources
- EPA, A Brief Guide to Mold, Moisture and Your Home — molds grow on wet indoor surfaces; control moisture to control mold; small-area DIY versus professional thresholds.
- CDC, Mold: Basic Facts — remove indoor mold regardless of type; no separate "mildew" health standard.
- EPA, Mold and Health — respiratory and allergic effects in sensitive individuals.
- WHO, Guidelines for Indoor Air Quality: Dampness and Mould — dampness raises respiratory-symptom risk.