  FU Orionis (FUor) and EX Lupi (EXor) type objects represent two small, but
rather spectacular groups of low-mass, young eruptive stars. Outbursts of
several magnitudes are observed, attributed to enhanced accretion from the
circumstellar disk onto the central protostar. The host molecular environments
of FUors/EXors are poorly explored due to the scarcity of systematic molecular
line observations. We carried out the first dedicated survey of the molecular
environments of a large sample of FUors/EXors, observing a total of 51 sources
with the aim of studying the ammonia (NH$_3$) emission in their host
environments. We observed the ammonia (J,K)=(1,1), (2,2), and (3,3) inversion
transitions using the Effelsberg 100-m radio telescope. We derived H$_2$ column
densities and dust temperatures using archival Herschel SPIRE data. We detected
the (1,1) transition toward 28 sources and the (2,2) transition toward 12
sources, while the (3,3) transition was detected toward only two sources. We
find kinetic temperatures between ~12 K and 21 K, ammonia column densities from
$5.2\times10^{13}\,cm^{-2}$ to $3.2\times10^{15}\,cm^{-2}$, and fractional
ammonia abundances with respect to H$_{2}$ from $4.7\times10^{-9}$ to
$1.5\times10^{-7}$. The results are comparable to those found in infrared dark
clouds (IRDCs). Kinetic analysis suggests that most of the eruptive stars in
our sample reside in rather quiescent (sonic or transonic) host environments.
Our NH$_3$ observations and analysis of the SPIRE dust-based H$_2$ column
density maps confirm the presence of dense material toward 7 sources in our
sample; additional sources might also harbour dense gas based on their NH$_2$
(2,2) detections, might indicate an earlier phase than originally classified.
Based on our results, we suggest observations targeting additional molecular
lines would help to refine the evolutionary classification of eruptive stars.
