Calculate the $8-$hour time$-$weighted average $($TWA$)$ exposure to

wood dust for

Working period Wood cutting task Exposure mg$/$m$3$

$08$:$0009$:$00$ Oak fire door $2\mathrm{.}9$

$09$:$3010$:$15$ Oak floorboards $3\mathrm{.}8$

$10$:$4511$:$15$ Pine floorboards $6\mathrm{.}0$

$13$:$3015$:$30$ Plywood sheets and mahogany stair rails $4\mathrm{.}6$

$15$:$4516$:$45$ Chipboard sheets $5\mathrm{.}0$

$17$:$0017$:$45$ Pine floorboards $4\mathrm{.}6$