Fallout vs. Fall Out
“Fallout” is a closed noun with meanings including radioactive dust and adverse consequences. “Fall out” is a two-word phrasal verb with different verb senses, including becoming detached and quarrelling.
Use “fallout” as a noun for radioactive dust or the unpleasant consequences of an event. Use “fall out” as a phrasal verb for becoming detached or for having an argument and no longer being friendly.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| radioactive particles or unpleasant consequences | fallout (one-word noun) | Use the closed noun. |
| become detached | fall out (phrasal verb) | Use the spaced verb. |
| have an argument and stop being friendly | fall out (phrasal verb) | Use the spaced verb. |
Quick tests
Meaning / construction test
Does the expression function as a verb that can change tense or inflect in the sentence?
If yes: Use “fall out.”
If no: If it names radioactive material or consequences, use the noun “fallout.”
Caveat: The noun and phrasal verb have multiple documented senses; do not reduce the distinction to one universal noun/verb conversion beyond those senses.
Examples
The political fallout continued for months.
“Fallout” is a noun meaning unpleasant consequences.
A loose screw may fall out during transport.
“Fall out” is a phrasal verb meaning become detached.
They fell out over the decision.
The inflected phrasal verb describes an argument and damaged relationship.
Further guidance
Core difference
Use the documented noun or modifier form when naming a thing, result, event, or concept; use the spaced form when the words function as a verb.
Fast Test
If the expression can take ordinary verb tense or inflection in the sentence, the spaced verb form is usually the relevant choice.
Variant Boundary
Cambridge records closed “fallout” as a noun and “fall out” as a phrasal verb; the meanings are related only in spelling, not by one universal noun/verb conversion.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- FALLOUT — English meaning (opens in a new tab)Cambridge Dictionary · Closed noun “fallout”
- FALL OUT — English meaning (opens in a new tab)Cambridge Dictionary · Two-word phrasal verb “fall out”